Minoxidil, Minoxidil Sulfate, and Follicular Sulfotransferase

By Benn Bluestein-Veyra, M.Sc. Organic Chemistry · Updated 1 October 2026

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.^[1] 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.^[2] 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.

Minoxidil is a prodrug that the follicle has to activate

Minoxidil was developed as an oral antihypertensive, and hair growth was recorded as an adverse effect before it was studied as a topical agent, a history the 2019 review by Suchonwanit and colleagues in Drug Design, Development and Therapy sets out.^[1] The active species is minoxidil sulfate, produced from minoxidil by sulfotransferase enzymes, and in the scalp that conversion happens in the follicle itself (Fig. 1).^[1] A topical solution therefore depends on two things: the concentration applied and the enzymatic capacity of the follicles it reaches.

A key blank cut at the door is a fair analogy for the sequence. The bottle supplies the blank (Fig. 2a), and the follicle has to cut it before it turns anything (Fig. 2b).

Fig. 2 | Minoxidil and Minoxidil sulfate.

Follicular enzyme activity predicts response on the scalp and by mouth

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 (Fig. 3) predicted which women with pattern hair loss responded to topical minoxidil,^[3] 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 (Fig. 4) predicted response to oral minoxidil.^[4]

Jimenez-Cauhe and colleagues reported in 2024 on follicular sulfotransferase activity and the effectiveness of oral minoxidil in androgenetic alopecia.^[5] Randolph and Tosti reviewed the oral route in the Journal of the American Academy of Dermatology in 2021, and that review gives the assay papers their context.^[6]

The literature reports that the enzyme can be measured in a plucked follicle and, in some studies, induced.^[3][7] That is a finding about the enzyme, and it makes no claim about any product.

Tretinoin induces the enzyme, and the pairing predates the mechanism

Tretinoin (Fig. 5) enters the minoxidil story as an inducer of the enzyme. 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.^[7] Dhurat and colleagues reported a randomized trial of an enzyme-booster approach to topical minoxidil in the Journal of Cosmetic Dermatology in 2022.^[8] Those two studies are 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,^[9] and the combination sat in dermatology practice for three decades on an empirical footing before the sulfotransferase work explained why it might matter. Maas and colleagues reviewed the whole co-therapy literature in Skin Appendage Disorders in 2026.^[10] A reader comparing the 1986 report and the 2019 enzyme study is reading the same observation from opposite ends of a forty-year corridor.

Approved topical minoxidil and a compounded solution are separate products

Topical minoxidil at 2% and 5% is FDA-approved as an over-the-counter product,^[11][12] 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 randomized trials (Fig. 6).^[13] The 2022 network meta-analysis by Gupta and colleagues in JAMA Dermatology ranked minoxidil against the 5-alpha-reductase inhibitors in men.^[14] 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.^[2] Where a compounded solution contains finasteride, the FDA alert on compounded topical finasteride, discussed on the finasteride page of this cluster, applies to it.^[15]

Each ingredient has a rationale, and the four-ingredient combination has no trial

The table separates two things that marketing tends to merge. Each ingredient has a rationale in the literature,^{[7][16][17][18]} and for two of them the literature includes randomized trials of a two-ingredient solution.^[8][19] 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.

A formulary reader can take five points from this literature

• Minoxidil is a prodrug; its active form is made in the follicle by sulfotransferase, and enzyme activity varies between people.^[1]
• Follicular sulfotransferase activity has been reported to predict response to topical minoxidil, and SULT1A1 variants to predict response to oral minoxidil.^[3][4]
• Tretinoin has been reported to induce the enzyme, which is the rationale for the minoxidil and tretinoin solution.^[7]
• Approved topical minoxidil is an over-the-counter product;^[11][12] 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.

Read this way, the literature explains why two patients using the same bottle can diverge. It also keeps the approved product and the compounded solution in separate regulatory boxes.

A longer ingredient list adds rationales of very different strengths, and the literature measures each one separately. Four component rationales describe four studies, and the combination itself still awaits a trial.