Why trust NCOA

Key Takeaways

• Some hair loss serums contain ingredients scientifically proven to promote hair growth, like minoxidil. 
• Our top hair loss serum brands include Hims, Hers, and Nutrafol since our testers experienced good results with these products. 
• Prices for the hair loss serums and kits in our roundup range from $13–$150. 
• Hair serums come in liquid and foam versions. Liquids may be easier to apply, but foams may be less irritating to sensitive scalps.

Hair loss can happen for a number of reasons, but there are many treatments available today, including hair loss medications and over the counter options. “There is science behind some hair loss serums, though it’s important to note that not all serums are created equal,” said Ross Kopelman, MD, hair transplant surgeon at Kopelman Hair Restoration. 

Based on our research and conversations with experts, we identified several important factors that contribute to a quality hair loss serum. We reviewed eight hair loss serums that may help regrow thinning hair. Here are our expert thoughts.

Best hair loss serums in 2024

• Nutrafol Women’s Hair Serum: Best Overall
• Hers: Best for Women
• Hims: Best for Men
• Happy Head TopicalRx Solution: Best Prescription 
• Vegamour GRO Hair Serum: Best Plant-Based
• Act + Acre Advanced Fuller Hair System: Best Hair System
• Divi Hydrating Starter Bundle: Best Shampoo and Serum
• Philip Kingsley Density Preserving Scalp Drops: Best for Hair Shedding

What we look for in serums for hair growth

Nutrafol: Best Overall Hair Serum

Our expert take on Nutrafol Hair Serum

Nutrafol offers a full range of products designed to promote thicker hair. Our testers enjoyed using the hair serum. They said it wasn’t greasy and did not make their scalps feel oily afterward, unlike some other serums. It was also easy to incorporate into their regular hair care routine after a shower. One tester noticed a difference from using the serum and the Nutrafol Nutraceutical after one month and said she sees a lot of new baby hairs growing.

Nutrafol hair serum doesn’t contain prescription medications, like minoxidil, so it can be a good option for those who want to treat their hair loss with natural ingredients. Instead, the hair serum’s active ingredient is the herbal ashwagandha, which some research suggests can help with hair growth. One study found that a topical ashwagandha serum led to “substantially enhanced hair density, growth, and thickness” compared to a placebo. You will need to keep using the serum to see the benefits. Once you stop using the product, your previous hair patterns could return. 

Our tester on the Nutrafol Hair Serum and Shampoo:

“I absolutely loved Nutrafol’s hair serum. I put it on my scalp after the shower, and it was great because it doesn’t make your scalp oily at all, so you don’t need to worry about only using it on hair wash days.”

Our expert take

Who may benefit

• People who want a hair loss serum that doesn’t use prescription medications 
• People who want to treat hair loss with vegan ingredients
• People who want a hair serum that does not leave their hair greasy 

Who may consider other options

• People who want a prescription medication for hair loss

Customer reviews

On Amazon, Nutrafol Hair Serum has 4.2 out of five stars with 2,800 total reviews. Many reviewers report seeing thicker hair from using the product consistently, although some reviewers note that the formula may smell unpleasant due to the lack of fragrances. 

“​​I’ve been using the Nutrafol Women’s Hair Serum for a year now, and it’s been a game-changer. As a mom of a 2-year-old dealing with postpartum shedding, this serum has really helped manage the hair loss. The formula is lightweight and doesn’t weigh my hair down, making it easy to use daily. I highly recommend it for anyone going through a similar experience.”

– Keya, verified buyer, Aug. 20, 2024

“The product itself works fantastic. It must be used consistently for 3 months or longer to actually see a genuine difference. […] The serum is thin and a bit runny so you don’t need much to spread it around the scalp. If you are treating more than a few thin spots and need something for overall scalp treatment, this serum is going to be used up very fast. And it is something you will always need to use or the follicles that were originally dormant will eventually go dormant again. That gets expensive.”

– Sue, verified buyer, Feb. 12, 2024

Hers Minoxidil Foam for Women: Best Hair Loss Serum for Women

Our expert take on Hers Minoxidil Foam for Women

The Hers Minoxidil Foam is designed exclusively to treat hair loss in women. Its main active ingredient is minoxidil, which is FDA-approved for treating both male and female pattern hair loss in topical form. 

Hers makes both liquid and foam minoxidil products, but we chose the foam version, which contains 5% active minoxidil. Our testers said the foam felt light and non-greasy when applied to wet hair and found it easy to massage into the scalp. They followed the packaging directions and used the treatment once per day for three months. Our testers experienced thicker hair along their hairline and were pleased with their results.

Testers also said they liked how easy it was to add the foam to their daily routines because it takes about one minute to apply. One tester said the foam left some residue when they applied it to dry hair, which could be an issue if you do not wash your hair every day.

Our tester on Hers:

“I was worried that the foam would have some sort of negative effect on my hair–whether making it look greasy, leaving behind an uncomfortable residue, or something else. However, the foam was light and airy, incorporated easily into my hair, and wasn’t even noticeable after letting it soak in.”

Our hair loss expert’s take

Who may benefit

• People who want a foam hair loss product instead of a liquid
• Someone who wants a once-daily topical hair loss treatment 
• People who prefer a subscription for their hair loss treatment  

Who may consider other options

• People with especially sensitive scalps, since the product contains alcohol, which may be drying

Customer reviews

Hers Minoxidil Foam for Women has a 4.5 out of five-star rating with 65 total reviews on
its website. On Amazon, the product has a 4 out of five-star rating with nearly 1,900 total reviews. 

“I can see a difference, especially after using both bottles. It is a better value to buy 2 at a time. I highly recommend this product for women who are seeing thinning hair. The volume is definitely better.”

– Christine V., verified buyer, Oct. 2, 2024

“It took several months but it does work. I am seeing results. The downside is, it tells you not to get it on your hair. For me that’s pretty impossible. End result, it makes my hair feel so dirty. I’m torn to keep using it and have my hair always seem dirty. So I settled just for using it every other day or so.”

– Teresa, verified buyer, Sept. 18, 2024

Read our Hers hair loss review for more information.

Hims Rx Hair Loss Spray: Best Hair Loss Serum for Men

Our expert take on Hims Rx Hair Loss Spray

Hims hair loss serum is available as either a liquid serum or a spray. Both are only available by prescription for men because they contain the active ingredients minoxidil and finasteride. To see if you qualify for a hair loss prescription, you can complete an intake and medical assessment online directly through Hims. If you’re a good candidate for a prescription Hims hair loss product, you get free shipping on your hair loss serum or spray and unlimited virtual follow-ups with a medical professional.

Our testers tried the Rx Hair Loss Spray version, along with the shampoo and biotin supplement. They liked how easy it was to apply the spray, though one tester noticed that the spray bottle nozzle could clog after a few weeks of use. Testers said they saw thicker hair even when they weren’t using the products consistently.
Hims hair loss serum and spray are compounded medications, meaning that the two medications are mixed together and aren’t FDA-approved. If you’d prefer an FDA-approved hair loss medication, Hims also offers generic minoxidil and finasteride, which are available separately. There are some potential side effects from topical finasteride, including scalp irritation, itching, burning sensation, flaking, and redness where it is applied. It is a good idea to talk to your doctor about possible medication interactions or complications before starting any new prescription.

Our tester on the Hims Rx Hair Loss Spray:

“I have noticed my hair is getting a little thicker at the top. My bald spot is still there, but I haven’t been consistent with application, so that’s on me.”

Our hair loss expert’s take

Who may benefit

• Men who want a prescription medication hair loss serum 
• Men who want to try minoxidil and finasteride together
• Men with advanced hair thinning or hair loss

Who may consider other options

• Men who want an over-the-counter hair loss serum
• Men who are uncomfortable using finasteride

Customer reviews

Hims Rx Hair Loss Spray has 2.9 out of five stars on the Hims website with more than 1,670 reviews. Positive reviews say the product worked for them and they saw results in about three months. Negative reviews mention feeling some side effects on the scalp, like redness and irritation, and multiple reviews say the spray bottle itself gets clogged easily and stops working. 

“Very impressed with the product. I had visible results in less than three months. I was skeptical that it would work, but I am happily surprised.”

– Drew, verified buyer, Oct. 10, 2024

“I think the product is working, but the spray bottles don’t work. When I try to press the button down it’s locked. It happened with both bottles so that is frustrating.”

– James, verified buyer, Aug. 27, 2024

Read our Hims hair loss review for more information.

Happy Head TopicalRx Solution: Best Prescription Hair Loss Serum

Our expert take on the Happy Head TopicalRx Solution

Happy Head TopicalRx Solutions are available by prescription for men over 18 and women over 50. Happy Head provides consultations with a board-certified dermatologist to determine if you’re a good candidate and can write your prescription. This process involves an initial questionnaire to provide information about your hair loss and medical history. Your information is reviewed by a dermatologist, who will then meet with you on a video call to discuss your options. These hair serums are available in formulas with different prescription medications for hair loss like minoxidil, finasteride, spironolactone, and dutasteride. Dutasteride is a medication that blocks the hormone dihydrotestosterone (DHT), a contributing factor to hair loss. One research review found that an oral version of dutasteride was better at suppressing DHT than oral finasteride. The company can customize your serum based on your type of hair loss and preferences, with 180 different possible combinations.

While each of these medications is FDA-approved for hair loss, when combined in a Happy Head serum with other ingredients they are considered compounded medications. The serums themselves are not FDA-approved. These medications may also cause side effects or interact with other medications, so be sure to talk to your health care provider before using a prescription serum. Women who are pregnant or may become pregnant should not use finasteride due to a risk of birth defects. Some of the serums also contain alcohol, which experts say can be drying on the scalp.

Our tester on the Happy Head TopicalRx Solution:

“More of a liquid than a serum. Might leave a slight grease because I could feel the residue on my hands but it is not super thick so seems lightweight.”

Our hair loss expert’s take

Who may love it

• People who want a custom-formulated hair loss serum due to sensitivity to certain medications
• Those who want a prescription hair loss serum for more advanced hair loss 

Who may want to avoid it

• People who prefer an over-the-counter hair serum to avoid fewer potential side effects
• Women under age 50 who cannot take finasteride

Customer reviews

Happy Head has 4.3 out of five stars on Trustpilot with more than 985 reviews. Positive reviews say the products work and users report seeing results from the prescription medication products after several months of use. They also say they had good experiences with Happy Head customer service. Numerous negative reviews mention that it was difficult to cancel their subscription, while several reviewers questioned if the custom serums were safe to use if they were clinically tested.

“I have had a great experience with Happy Head. I have tried other products and they irritated my scalp. I have been using the Topical Finasteride & Minoxidil for over a year and I can see a dramatic improvement. Once my friends and family started to make comments I knew that the product was working well.”

– Chris W., verified buyer, Oct. 4, 2024

“Topical product left my hair dry and crusty after each use. Went to cancel my subscription and after making me fill out a bunch of information on their website I received a message that said “If you decide to proceed with the cancellation, please call us.”

– Arin B., verified buyer, June 29, 2024

Vegamour GRO Hair Serum: Best Plant-Based Serum for Hair Loss

Our expert take on Vegamour GRO Hair Serum

The main ingredient in the GRO hair serum is caffeine, which some research suggests can help with hair growth. “Studies suggest that caffeine can help counteract the effects of DHT, the hormone responsible for hair loss in androgenetic alopecia,” said Dr. Kopelman. 

Vegamour conducted a 150-day consumer study of 20 participants using the GRO hair serum daily; 95% of participants reported less hair shedding, and 83% said their hair felt stronger. While Vegamour claims that there are no known long-term side effects of their ingredients, some evidence suggests that using too much caffeine on your hair can actually decrease hair growth. It’s important to follow instructions and only apply the serum once per day. 

Our hair loss expert’s take

Who may benefit

• People looking for a hair loss serum without prescription medications
• Vegetarians or vegans who want plant-based active ingredients

Who may consider other options

• People on a tight budget, as a monthly subscription costs $49

Customer reviews

On the Vegamour website, the GRO Hair Serum has 4.4 out of five stars. Many reviewers report good results and thicker hair, but some stated that they used the serum alongside other hair regrowth products, like minoxidil. Others didn’t see any improvements with the serum. 

“I’m only a month in but can tell my hair is shedding less, and it just looks healthier. Very happy to have found a topical that makes a difference!”

– Nicole M., verified buyer, Sept. 5, 2024

“I have always had thin/thinning hair… I started this product and saw results very quickly… I recommend this to everyone who complains about their hair.

– Angelica G., verified buyer, Aug. 24, 2024

Act + Acre Advanced Fuller Hair System: Best Hair Loss System

Our expert take on the Act + Acre Advanced Fuller Hair System

The Act + Acre Advanced Fuller Hair System combines three products into one kit, which you can purchase once or as a subscription that delivers every three months. The hair serum contains apple stem cells, bamboo and pea sprout blend, and hyaluronic acid, which the company claims can promote thicker and fuller-looking hair. One 2019 study found that human stem cells were able to improve hair growth when injected into the scalp, but more research is needed to determine if similar effects are possible with plant-based stem cells applied topically. According to a survey of 142 users by Act + Acre, nearly 90% of respondents said their scalp looked healthier after using the Stem Cell Scalp Serum. It’s important to note that an improvement in the appearance of the scalp does not necessarily mean the serum can help regrow or thicken hair.

In addition to the hair serum, the hair system also contains a scalp detox, which you apply before washing your hair, and twice-daily capsules containing vitamins A, C, D2, B1, B6, B12, and other ingredients, like biotin. The kit may be convenient if you’d like to combine your hair loss serum with other products. But it’s also on the more expensive side, with a one-time purchase costing $150. This kit may be a good option if you have scalp dryness and irritation or hair thinning, but may not be right for more advanced types of hair loss.

Our expert take

Who may love it

• People who want to use a hair loss serum as part of a full hair care system
• Those who want a vegan-friendly hair loss serum
• People who have scalp dryness and irritation or thinning hair

Who may want to avoid it

• Shoppers on a budget, as the full system can be expensive even with a subscription
• People who want prescription medication for hair loss

Customer reviews

The Act + Acre Advanced Fuller Hair System has 4.7 out of five stars with 1,840 total reviews on the company website. This rating combines reviews from the products included in the kit. Several reviewers mentioned seeing stronger hair and new growth with the serum, but others didn’t see any results.

“This serum does wonders for my dry scalp! My hair feels fuller and healthier when I use it regularly. I took a break for a while and came back, and instantly noticed a difference. I will be keeping this stocked from now on.”

– Michaela, verified buyer, Sept. 25, 2024

“I’ve been using this product for a year and my hair feels thicker, like there are more hairs growing because of the reduced shedding. It doesn’t make my hair greasy at all. I’ve gotten so many compliments on my hair and I always end up sharing this product.”

– A. Rad, verified buyer, Aug. 15, 2024

Divi Hydrating Starter Bundle: Best Hair Loss Shampoo and Serum

Our expert take on the Divi Hydrating Starter Bundle

Divi may be a good choice if you’d like a hair serum to help you maintain a healthy scalp. It contains caffeine as a key ingredient, which some research shows can improve hair growth. You should be cautious not to overtreat hair with caffeine as it may actually decrease hair growth. It also contains ingredients like hyaluronic acid and tea tree oil to moisturize the scalp and amino acids to fight hair breakage.

In a survey of people who used the serum for 12 weeks, 90% of the participants said they saw fuller hair, and 82% said they experienced less scalp dryness. While Divi says the serum doesn’t have any known side effects, some people may have sensitivities to the ingredients. For example, tea tree oil can cause allergic reactions in some people, so use with caution if you have any known skin sensitivities.  

You can purchase the serum as a one-time purchase online, or you can purchase it as part of the Hydrating Starter Bundle. This bundle also comes with the Divi Hydrating Shampoo and Divi Hydrating Conditioner to help moisturize and soften the hair.

Our hair loss expert’s take

Who may love it

• People who want a hair loss serum that supports a healthy scalp
• Those who want to bundle a hair loss serum with a hair loss shampoo

Who may want to avoid it

• People with sensitivities to tea tree oil or any of the other ingredients
• Those with advanced or severe hair loss, as the serum may not support full hair regrowth 

Customer reviews

The Divi Scalp Serum has 4.6 out of five stars on the Divi website with more than 12,600 total reviews. Many reviewers report thicker hair and a healthier scalp, while others didn’t see a noticeable difference.  

“I love this scalp serum. I have had WAY less shedding and breakage and my hair and scalp feel so healthy!”

– Emily, verified buyer, Aug. 9, 2024

“This serum is amazing! My hair was thinning so much in the front and I started using this about a year ago. Night and day difference! So happy I found Divi!”

– Keri L, verified buyer, Aug. 6, 2024

Philip Kingsley Density Preserving Scalp Drops: Best hair loss serum for shedding

Our expert take on Philip Kingsley Density Preserving Scalp Drops

If your main concern is hair shedding, Philip Kingsley Density Preserving Scalp Drops could be a good place to start. The brand states that in an independent clinical trial the scalp drops reduced hair shedding after three months of use. In a different independent user trial, three out of four users said their hair was visibly denser after using the drops for three months.

The drops have azelaic acid as a key ingredient, which one study found may promote hair growth when used in combination with minoxidil. But more research is needed to determine if azelaic acid can provide these effects on its own. The scalp drops also include other ingredients meant to promote hair growth, like caffeine. 

You can purchase the scalp drops as a one-time purchase or as a subscription, which includes an additional 20% off for your first order and 15% off for subsequent orders. You can also order a three-month supply for an additional 20% off.

Our hair loss expert’s take

Who may love it

• People worried about shedding hair
• Those who want a drug-free hair loss serum
• People who want thicker hair

Who may want to avoid it

• Shoppers on a budget, as a one-time purchase costs $70
• People who want an FDA-approved hair loss treatment

Customer reviews

The Philip Kingsley Density Preserving Scalp Drops have a 3.5 out of five-star rating on the website with 24 total reviews. While some reviewers report improvements to their hair, others say it makes their hair greasy and doesn’t impact their shedding. 

“I have been using these drops for a few months now. I shower 2 times a week and use them only after I shower, so not everyday, and they still work perfectly! The hair fall completely disappears if I use it constantly. And it comes back if I stop using them for a while.”

– Wedd, verified buyer, May 29, 2024

“I am a woman of a certain age (menopause) and have underactive thyroid. As a combination of these factors I am aware of the risk of hair thinning especially around my front hairline. I have been using these drops together with the density stimulating scalp toner… I think this generally has helped with hair loss (my hairdresser has commented too) and I am noticing small baby hairs coming through.”

– Clare P, verified buyer, July 29, 2023

How we test hair loss serums

Our Reviews Team bases our recommendations on the best clinical guidelines and research on hair loss treatments. We talked with dermatologists and trichologists to get their expert recommendations for both prescription and over-the-counter products. We also surveyed hair loss treatment users and interviewed focus groups of users to better understand the experience of hair loss and hair loss treatment. 

Our testers signed up for some of the most popular online hair loss treatment platforms, paying special attention to:

• Treatments available, including prescription and over-the-counter options
• Ways to contact a prescriber, for example through messaging or video sessions  
• The medical assessment and intake process, if prescribed a medication
• Shipping times
• Costs of subscriptions, products, and consultations 
• How easy it is to use the app or website
• Treatment experience
• Customer service

All of our reviews are medically reviewed by a licensed and practicing medical expert to ensure the information is accurate and up to date. Learn more by reading our full hair loss review methodology.

What to consider when purchasing a serum for hair growth 

While one serum can be ideal for many people, it may not be the right choice for your specific preferences. So keep the following factors in mind when narrowing down your choices. 

Ingredients 

Look for a hair serum with ingredients scientifically proven to support hair growth: 

• Minoxidil: This medication was the first FDA-approved treatment for hair loss and is available over the counter for topical use. 
• Finasteride: This prescription is the second FDA-approved hair loss medication, but it’s only approved for male-pattern baldness. 
• Caffeine: Some evidence suggests caffeine can stimulate hair follicles and promote hair growth. 
• Peptides or growth factors: “Serums containing peptides and growth factors may promote a healthier scalp environment, which can support hair growth,” said Dr. Kopelman. 

On the other hand, some hair serums also include ingredients that Dr. Han explained may be irritating to the scalp, like alcohol or sulfates. “[These] can promote further loss and breakage of existing hairs,” she said. If you have a sensitive scalp, look for a hair loss serum that doesn’t contain alcohol, or that only has a small amount. 

Oil vs. foam serums for hair loss 

Some brands on our list, like Hers, offer foam and oil-based hair loss serums. Which option you choose depends on your personal application preferences. But if you have a sensitive scalp and want to try a serum containing minoxidil, foam may be a better choice. Regular strength oil-based minoxidil contains solvents, like propylene glycol, which can cause scalp irritation. On the other hand, foam minoxidil solutions were developed to be propylene glycol-free, so they may be less irritating to sensitive skin. 

Frequency of application 

Every serum has its own directions for use, but many of the options in our roundup suggest applying once per day for at least three months to see results. 

What is the difference between a hair loss serum and a supplement? 

Some hair loss medications, like minoxidil, are available in oral and topical forms. While a hair loss serum may contain some of the same ingredients as an oral supplement, its end goal is slightly different. 

“Serums work topically, directly on the scalp, and can be effective for improving the local environment where hair follicles reside,” Dr. Kopelman explained. “On the other hand, supplements work internally, addressing potential nutritional deficiencies or hormonal imbalances that contribute to hair loss.”  

Are hair loss serums as effective as supplements? 

Because hair loss serums and supplements can address different root causes of hair loss, Dr. Kopelman said that one isn’t necessarily more effective than the other. “In many cases, a combination of both a serum and a supplement is recommended to maximize the potential for hair regrowth,” he explained.

Table 1, Compare the best serums for hair loss in 2024

*Price for bundle including hair serum.

Bottom line

Using a hair loss serum with science-backed ingredients may help you promote thicker hair and scalp health. Start with serums from brands like Hims or Hers and Nutrafol, since they contain science-backed ingredients and our testers experienced positive results while using them. When choosing the best hair serum for you, pay attention to the ingredient list, and pick an option that’s easy to apply so that you can work it into your regular hair care routine. 

Frequently asked questions

Do hair loss serums really work?

Many over-the-counter and prescription hair loss serums contain ingredients that are scientifically researched for hair growth, like minoxidil and caffeine, but your results with a serum may vary.

How do you apply hair loss serums?

While every option on our list has slightly different instructions, you’ll generally apply the serum on areas of concern, like the hairline, and massage it into the scalp. Consult your hair serum’s directions for more guidance.

Which hair loss serum is best for hair regrowth and thickness?

There isn’t a single “best” hair loss serum for everyone. Beyond choosing science-backed ingredients, Dr. Han suggests opting for a serum that you enjoy using, so it’s easier to incorporate it into your regular hair care routine.

Have questions about this review? Email us at reviewsteam@ncoa.org.

Sources

1. Williams R, et al. Getting under the skin of hair aging: the impact of the hair follicle environment. Experimental Dermatology. May 2, 2020. Found on the internet at https://onlinelibrary.wiley.com/doi/full/10.1111/exd.14109 
2. Godman, H. It’s not too late to save thinning hair. Harvard Health Publishing. Jan. 1, 2023. Found on the internet at https://www.health.harvard.edu/diseases-and-conditions/its-not-too-late-to-save-thinning-hair
3. Patel P, et al. Minoxidil. StatPearls. Sept. 1, 2024. Found on the internet at https://www.ncbi.nlm.nih.gov/books/NBK482378/ 
4. Yerram C, et al. Effects of Withania somnifera root extract serum application on hair health in healthy adults: A prospective, double-blind, randomized, parallel, placebo-controlled study. Journal of Ayurveda and Integrative Medicine. Nov. 24, 2023. Found on the internet at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10709127/
5. Visconti, MJ et al. Therapeutic Use of Caffeine in Dermatology: A Literature Review. Journal of Dermatology and Dermatologic Surgery. January-June 2020. Found on the internet at https://journals.lww.com/jdds/fulltext/2020/24010/therapeutic_use_of_caffeine_in_dermatology__a.4.aspx 
6. Compounding and the FDA: Questions and Answers. U.S. Food and Drug Administration. Found on the internet at https://www.fda.gov/drugs/human-drug-compounding/compounding-and-fda-questions-and-answers
7. Estill MC, et al. Finasteride and Dutasteride for the Treatment of Male Androgenetic Alopecia: A Review of Efficacy and Reproductive Adverse Effects. Georgetown Medical Review. Oct. 27, 2023. Found on the internet at https://gmr.scholasticahq.com/article/88531-finasteride-and-dutasteride-for-the-treatment-of-male-androgenetic-alopecia-a-review-of-efficacy-and-reproductive-adverse-effects 
8. Gentile P, et al. Stem cells from human hair follicles: first mechanical isolation for immediate autologous clinical use in androgenetic alopecia and hair loss. Stem Cell Investigation. Jun 27, 2017. Found on the internet at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5504091/
9. Ambrogio F, et al. Spreading Allergic Contact Dermatitis to Tea Tree Oil in an Over-the-Counter Product Applied on a Wart. Medicina. April 19, 2022. Found on the internet at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9146230/ 
10. Amirfakhryan E, et al. Azelaic acid stimulates catalase activation and promotes hair growth through upregulation of Gli1 and Gli2 mRNA and Shh protein. Avicenna Journal of Phytomedicine. September 2020. Found on the internet at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7508322/
11. Minoxidil: Formulation, Dosage & Side-Effects. International Society of Hair Restoration Surgery.  Found on the internet at https://ishrs.org/patients/treatments-for-hair-loss/medications/minoxidil/ 
12. Zito PM, et al. Finasteride. StatPearls. Feb. 8, 2024.  Found on the internet at https://www.ncbi.nlm.nih.gov/books/NBK513329/