Why is minoxidil described as a prodrug?

Because the molecule applied to the scalp is inactive. Sulfotransferase enzymes in the outer root sheath of the follicle convert minoxidil to minoxidil sulfate, which is the form that acts on the follicle. The amount of that enzyme varies between people, and a 2020 study by Ramos and colleagues reported that enzyme activity measured in plucked follicles predicted who responded to topical minoxidil.

Why is tretinoin combined with minoxidil in compounded solutions?

A 2019 study by Sharma, Goren and colleagues reported that five days of topical tretinoin raised follicular sulfotransferase expression and converted 43% of predicted minoxidil non-responders into responders. That enzyme-induction finding is the rationale for the minoxidil and tretinoin solution. The pairing itself was reported by Bazzano and colleagues in 1986, before the mechanism was described.

Is a compounded minoxidil solution the same as the approved over-the-counter product?

No. Topical minoxidil at 2% and 5% is FDA-approved as an over-the-counter product. A compounded solution that adds tretinoin, finasteride, azelaic acid or ketoconazole is prepared under section 503A to an individual prescription, carries a beyond-use date rather than a shelf life, and has not been evaluated by the FDA. Where it contains finasteride, the FDA alert on compounded topical finasteride applies.

Is there a trial of the minoxidil, azelaic acid, finasteride and ketoconazole solution?

No trial of that four-ingredient combination has been published. Two of its ingredients, finasteride and minoxidil, have randomised trials of a two-ingredient solution behind them, pooled in a 2025 meta-analysis. Ketoconazole and azelaic acid have shampoo studies and in vitro work respectively. The component studies do not amount to evidence for the combination.

Does ExaVeyra say which minoxidil solution a practice should use?

No. ExaVeyra is a supplier and takes no part in prescribing decisions. The compounded hair and scalp formulations page lists what the 503A pharmacy partner can prepare and how each solution is dated. Which solution, if any, suits a given patient rests with the prescriber.

Hair and scalp evidence

Minoxidil, Minoxidil Sulfate, and Follicular Sulfotransferase

Reviewed by ExaVeyra Sciences editorial team · Updated 15 September 2026

ExaVeyra Sciences is a Miami-based B2B supplier of research-use exosomes to licensed clinics, and it refers licensed prescribers to a 503A compounding pharmacy that prepares formulations against individual prescriptions.

What this page covers, who can buy from ExaVeyra, and what is supplied
Question	Answer
What this is	Why minoxidil is a prodrug, what follicular sulfotransferase activity predicts, how tretinoin enters the picture, and what the combination solutions contain.
Who can buy	Licensed clinics and practitioners, after ExaVeyra verifies an active professional license and NPI and, for exosome material, a signed research-use acknowledgment.
What ships or is provided	Research-use exosome formats and PRP devices ship from ExaVeyra through one verified account; compounded formulations are prepared and dispensed by our 503A pharmacy partner against a patient-specific prescription and do not ship from ExaVeyra. A certificate of analysis is issued for every lot and provided on request.

Minoxidil arrives at the hair follicle as an inactive molecule. Sulfotransferase enzymes in the outer root sheath convert it to minoxidil sulfate, the form that opens potassium channels and acts on the follicle, and the amount of that enzyme a given scalp carries varies widely between people. Topical minoxidil at 2% and 5% is FDA-approved as an over-the-counter product; the compounded solutions that pair it with tretinoin, or with finasteride, azelaic acid and ketoconazole, are prescription products prepared under section 503A. This page sets out the prodrug chemistry, the response literature built on it, and the rationale behind each combination.

It is an evidence reference. The compounded hair and scalp formulations page on this site lists the solutions the 503A pharmacy partner can prepare and how each is dated, and it deliberately says nothing about what any of them does to hair. Neither does this page.

A prodrug, and the enzyme that activates it

Minoxidil was developed as an oral antihypertensive, and hair growth was recorded as an adverse effect before it was studied as a topical agent. The active species is minoxidil sulfate, produced from minoxidil by sulfotransferase enzymes, and in the scalp that conversion happens in the follicle itself. A topical solution therefore depends on two things: the concentration applied and the enzymatic capacity of the follicles it reaches.

The second of those was the neglected variable for decades. The non-response rate to topical minoxidil was discussed in the literature in the context of adherence, formulation and disease severity, and a good part of it turned out to be enzymatic. Ramos and colleagues reported in Dermatologic Therapy in 2020 that sulfotransferase activity measured in plucked hair follicles predicted which women with pattern hair loss responded to topical minoxidil, and a 2021 study by the same group in the Journal of the European Academy of Dermatology and Venereology reported that genetic variants of the SULT1A1 enzyme predicted response to oral minoxidil.

Tretinoin as an enzyme inducer

Tretinoin enters the minoxidil story as an inducer of the enzyme rather than as a hair molecule in its own right. Sharma, Goren and colleagues reported in Dermatologic Therapy in 2019 that five days of topical tretinoin raised follicular sulfotransferase expression, and that 43% of subjects predicted by the enzyme assay to be minoxidil non-responders were converted to responders after that course. That study is the mechanistic basis for the minoxidil and tretinoin solutions in compounded formularies.

The pairing is older than the mechanism. Bazzano and colleagues reported on topical tretinoin with minoxidil in the Journal of the American Academy of Dermatology in 1986, and the combination sat in dermatology practice for three decades on an empirical footing before the sulfotransferase work explained why it might matter. A reader comparing the two papers is reading the same observation from opposite ends.

What the approved product is, and what a compounded solution is

Topical minoxidil at 2% and 5% is FDA-approved as an over-the-counter product, and the 2017 meta-analysis by Adil and Godwin in the Journal of the American Academy of Dermatology reported that it outperformed placebo on hair count across pooled randomised trials. That is the approved product and its evidence.

A compounded solution that adds tretinoin, or that adds finasteride, azelaic acid and ketoconazole to 5% minoxidil, is a different product. It is prepared under section 503A of the Federal Food, Drug, and Cosmetic Act to an individual prescription, it is dated with a beyond-use date rather than a shelf life, and it has not been evaluated by the FDA for safety, effectiveness or quality. Where a compounded solution contains finasteride, the FDA alert on compounded topical finasteride, discussed on the finasteride page of this cluster, applies to it.

The combinations, and the rationale behind each ingredient

Ingredient	Rationale in the literature	Evidence for the combination itself
Tretinoin 0.025%	Induces follicular sulfotransferase; converted predicted non-responders in a 2019 study	A 1986 report of the pairing; the 2019 enzyme study
Finasteride 0.1%	Type II 5-alpha-reductase inhibition in the follicle	A 2025 meta-analysis of seven trials of a mixed solution; a 2022 phase III trial of a 0.25% spray on its own
Ketoconazole 2%	Studied as a shampoo in a 1998 report; proposed as an adjunct to finasteride in a 2004 hypothesis paper	None for the four-ingredient combination
Azelaic acid 12.5%	Reported to inhibit 5-alpha-reductase in human skin in a 1988 in vitro study	None for the four-ingredient combination

The table separates two things that marketing tends to merge. Each ingredient has a rationale in the literature, and for two of them the literature includes randomised trials of a two-ingredient solution. No trial of the four-ingredient combination has been published, and a page that presented the component studies as evidence for the combination would be making a claim the studies do not carry.

Ketoconazole and azelaic acid have their own evidence page in this cluster, which takes the shampoo studies and the in vitro work at their own level of detail.

What a formulary reader takes from this

Minoxidil is a prodrug; its active form is made in the follicle by sulfotransferase, and enzyme activity varies between people.
Follicular sulfotransferase activity has been reported to predict response to topical minoxidil, and SULT1A1 variants to predict response to oral minoxidil.
Tretinoin has been reported to induce the enzyme, which is the rationale for the minoxidil and tretinoin solution.
Approved topical minoxidil is an over-the-counter product; every combination solution is a compounded prescription product with a beyond-use date.
No trial of the four-ingredient solution exists; its ingredients are supported individually and at different levels.

Sources

FDA, Compounding and the FDA: Questions and Answers (accessed 15 September 2026)
FDA alerts health care providers, compounders and consumers of potential risks associated with compounded topical finasteride products (April 22, 2025) (accessed 15 September 2026)
DailyMed, finasteride tablet 1 mg, prescribing information (National Library of Medicine) (accessed 15 September 2026)

Scientific literature

Each source carries the kind of study it was and, where the study enrolled people, how many. Study design decides what a result can establish, so it is stated rather than left to be inferred. Each line also says what that source is carrying on this page.

Ramos PM, Sinclair RD, Kasprzak M, Miot HA Sulfotransferase activity in plucked hair follicles predicts response to topical minoxidil treatment in Brazilian female pattern hair loss patients. Dermatologic Therapy, 2020. doi:10.1111/dth.13195 PMID:31846181Clinical studySource 1 supports: The report that follicular sulfotransferase activity predicted response to topical minoxidil, cited for the enzymatic explanation of non-response.
Ramos PM, et al. Minoxidil sulfotransferase enzyme (SULT1A1) genetic variants predicts response to oral minoxidil treatment for female pattern hair loss. Journal of the European Academy of Dermatology and Venereology, 2021. doi:10.1111/jdv.16765 PMID:32567076Clinical studySource 2 supports: The genetic counterpart of the enzyme-activity finding, cited to show the same mechanism reported for the oral route.
Sharma A, Goren A, Dhurat R, et al. Tretinoin enhances minoxidil response in androgenetic alopecia patients by upregulating follicular sulfotransferase enzymes. Dermatologic Therapy, 2019. doi:10.1111/dth.12915 PMID:30974011Clinical studySource 3 supports: The enzyme-induction study behind the minoxidil and tretinoin solution, cited for the 43% conversion of predicted non-responders.
Bazzano GS, Terezakis N, Galen W Topical tretinoin for hair growth promotion. Journal of the American Academy of Dermatology, 1986. doi:10.1016/s0190-9622(86)80024-x PMID:3771854Clinical studyLandmarkSource 4 supports: The original report of the tretinoin and minoxidil pairing, cited to show the combination predates the mechanism by three decades.
Adil A, Godwin M The effectiveness of treatments for androgenetic alopecia: a systematic review and meta-analysis. Journal of the American Academy of Dermatology, 2017. doi:10.1016/j.jaad.2017.02.054 PMID:28396101Meta-analysisSource 5 supports: The pooled randomised evidence for approved topical minoxidil against placebo on hair count.
Frontiers in Medicine (Lausanne), 2025 Comparing minoxidil-finasteride mixed solution with minoxidil solution alone for male androgenetic alopecia: a systematic review and meta-analysis of randomized controlled trials. Frontiers in Medicine, 2025. PMID:41127390Meta-analysis396 participantsSource 6 supports: The pooled evidence for the two-ingredient minoxidil and finasteride solution, cited in the combinations table.
Piérard-Franchimont C, De Doncker P, Cauwenbergh G, Piérard GE Ketoconazole shampoo: effect of long-term use in androgenic alopecia. Dermatology, 1998. doi:10.1159/000017954 PMID:9669136Clinical studySource 7 supports: The shampoo study that supplies the ketoconazole rationale in the combinations table.
Stamatiadis D, Bulteau-Portois MC, Mowszowicz I Inhibition of 5 alpha-reductase activity in human skin by zinc and azelaic acid. British Journal of Dermatology, 1988. doi:10.1111/j.1365-2133.1988.tb03474.x PMID:3207614In vitroSource 8 supports: The in vitro 5-alpha-reductase finding that supplies the azelaic acid rationale in the combinations table.
Suchonwanit P, Thammarucha S, Leerunyakul K Minoxidil and its use in hair disorders: a review. Drug Design, Development and Therapy, 2019. doi:10.2147/DDDT.S214907 PMID:31496654Narrative reviewSource 9 supports: The standard review of minoxidil pharmacology, cited for the prodrug chemistry and the sulfotransferase activation step.
Randolph M, Tosti A Oral minoxidil treatment for hair loss: a review of efficacy and safety. Journal of the American Academy of Dermatology, 2021. doi:10.1016/j.jaad.2020.06.1009 PMID:32622136Narrative reviewSource 10 supports: The review of the oral route, cited so the enzyme literature on oral minoxidil can be read in its context.
Dhurat R, Daruwalla S, Pai S, et al. SULT1A1 (minoxidil sulfotransferase) enzyme booster significantly improves response to topical minoxidil for hair regrowth. Journal of Cosmetic Dermatology, 2022. doi:10.1111/jocd.14299 PMID:34133836Randomised controlled trialSource 11 supports: A randomised trial of an enzyme-booster approach to topical minoxidil, cited as the controlled test of the induction rationale.
Jimenez-Cauhe J, Vano-Galvan S, Mehta N, et al. Hair follicle sulfotransferase activity and effectiveness of oral minoxidil in androgenetic alopecia. Journal of Cosmetic Dermatology, 2024. doi:10.1111/jocd.16473 PMID:39034734Clinical studySource 12 supports: The report relating follicular sulfotransferase activity to the oral route, cited to show the enzyme finding extends beyond topical use.
Gupta AK, Venkataraman M, Talukder M, Bamimore MA Relative efficacy of minoxidil and the 5-alpha reductase inhibitors in androgenetic alopecia treatment of male patients: a network meta-analysis. JAMA Dermatology, 2022. doi:10.1001/jamadermatol.2021.5743 PMID:35107565Meta-analysisSource 13 supports: The network meta-analysis ranking minoxidil against the 5-alpha-reductase inhibitors in men, cited for the comparative picture.
Maas D, Spindler A, Zappi I, et al. Efficacy of topical tretinoin and topical minoxidil cotherapy in androgenetic alopecia: a review. Skin Appendage Disorders, 2026. doi:10.1159/000553018 PMID:42535115Narrative reviewSource 14 supports: The 2026 review of the tretinoin and minoxidil pairing, cited for its survey of the co-therapy literature from 1986 to the enzyme studies.

Apply for wholesale access Ask our team

Common questions

Why is minoxidil described as a prodrug?: Because the molecule applied to the scalp is inactive. Sulfotransferase enzymes in the outer root sheath of the follicle convert minoxidil to minoxidil sulfate, which is the form that acts on the follicle. The amount of that enzyme varies between people, and a 2020 study by Ramos and colleagues reported that enzyme activity measured in plucked follicles predicted who responded to topical minoxidil.
Why is tretinoin combined with minoxidil in compounded solutions?: A 2019 study by Sharma, Goren and colleagues reported that five days of topical tretinoin raised follicular sulfotransferase expression and converted 43% of predicted minoxidil non-responders into responders. That enzyme-induction finding is the rationale for the minoxidil and tretinoin solution. The pairing itself was reported by Bazzano and colleagues in 1986, before the mechanism was described.
Is a compounded minoxidil solution the same as the approved over-the-counter product?: No. Topical minoxidil at 2% and 5% is FDA-approved as an over-the-counter product. A compounded solution that adds tretinoin, finasteride, azelaic acid or ketoconazole is prepared under section 503A to an individual prescription, carries a beyond-use date rather than a shelf life, and has not been evaluated by the FDA. Where it contains finasteride, the FDA alert on compounded topical finasteride applies.
Is there a trial of the minoxidil, azelaic acid, finasteride and ketoconazole solution?: No trial of that four-ingredient combination has been published. Two of its ingredients, finasteride and minoxidil, have randomised trials of a two-ingredient solution behind them, pooled in a 2025 meta-analysis. Ketoconazole and azelaic acid have shampoo studies and in vitro work respectively. The component studies do not amount to evidence for the combination.
Does ExaVeyra say which minoxidil solution a practice should use?: No. ExaVeyra is a supplier and takes no part in prescribing decisions. The compounded hair and scalp formulations page lists what the 503A pharmacy partner can prepare and how each solution is dated. Which solution, if any, suits a given patient rests with the prescriber.

Reference only. Not legal or medical advice.

The content of these guides is provided for general informational purposes only. It does not constitute legal, medical, or regulatory advice, and does not establish that any particular product or practice is compliant. Regulatory requirements vary by state and depend on the circumstances of each practice. Each practitioner should consult their own legal counsel and the applicable state licensing boards before acting on anything stated here.

Read the full disclaimer

ExaVeyra Sciences publishes these guides as a reference for licensed practitioners. They summarize publicly available federal and state requirements as those requirements read on the review date shown above. The content is provided for general informational purposes only and does not constitute legal, medical, or regulatory advice, and it does not establish that any particular product or practice is compliant.

No attorney-client relationship, and no professional or clinical relationship of any kind, is created by accessing these materials. ExaVeyra Sciences is a supplier of research-use materials. It is not a law firm, it is not a regulatory consultant, and it takes no part in any diagnosis, treatment decision, or plan of care. Each practitioner is responsible for retaining their own legal counsel and for determining, with that counsel and the applicable state licensing boards, how the requirements described here apply to the services, staffing, and structure of their own practice.

Statutes, regulations, board rules, and agency positions are subject to amendment, supersession, and reinterpretation. ExaVeyra Sciences makes no representation or warranty as to the accuracy, completeness, timeliness, or applicability of this content to any particular practice. Citations and links to third-party sources are provided for convenience only and do not constitute an endorsement of their content.

The practitioner and the practice are solely responsible for compliance with all applicable licensing, scope of practice, supervision, ownership, facility, recordkeeping, and tax requirements, and for all clinical decisions, including the suitability of any material for a given patient. To the fullest extent permitted by law, ExaVeyra Sciences disclaims all liability for any action taken or not taken in reliance on this content.