Minoxidil has one of the best safety records of any hair loss treatment, yet a large minority of patients who apply it diligently, twice daily, for six months or more, see little to no benefit. The usual assumption is poor compliance. Increasingly, the evidence points somewhere else entirely: an enzyme in the hair follicle itself.
Minoxidil is a Prodrug
Topical minoxidil is biologically inert as applied. Before it can act on the follicle, it must be converted into its active form — minoxidil sulfate — by a sulfotransferase enzyme, primarily SULT1A1, located in the outer root sheath of the hair follicle.1 No conversion, no effect, regardless of concentration, frequency, or duration of use.
This single step is the rate-limiting factor in topical response. Follicular SULT1A1 activity is not uniform across the population — it varies substantially between individuals, and that variation turns out to track closely with who responds to topical minoxidil and who does not.1
A patient who "doesn't respond to minoxidil" may simply lack the follicular enzyme needed to activate it — no amount of extra application will fix that.
What the Evidence Shows
Goren and colleagues developed an enzymatic assay, performed on a small sample of plucked hair follicles, that measures an individual's follicular SULT1A1 activity and predicts topical minoxidil response with high sensitivity.1 The existence of a validated predictive test is itself the clearest evidence that non-response has an identifiable biological cause, rather than being attributed by default to poor application technique.
Pooling this enzyme-based work with the broader clinical literature on topical minoxidil's real-world response rates, published estimates suggest that somewhere in the order of 30–40% of people have follicular enzyme activity low enough to produce a poor or absent response to topical minoxidil, independent of how consistently it is used.1,2
Why Oral Minoxidil Sidesteps the Problem
Oral minoxidil is also a prodrug, but it is activated in a different place: the liver, via hepatic sulfotransferase, as part of normal systemic metabolism. Because this activation happens before the drug ever reaches the scalp, it does not depend on the local enzyme environment of the hair follicle.3 A patient with low follicular SULT1A1 — who would see little from the topical form no matter how it was applied — can still generate active minoxidil sulfate reliably via the oral route.
A 2024 randomised clinical trial comparing oral and topical minoxidil in male androgenetic alopecia reported outcomes at least comparable to, and in several measures favouring, the oral formulation.4 Taken together with the enzyme data, this is the basis for recommending oral minoxidil as the first-line option for most patients here, rather than defaulting to topical and only escalating to oral after months of apparent treatment failure.
What This Means in Practice
- Topical minoxidil requires follicular enzyme activation; a substantial minority of people cannot make this conversion efficiently
- This is a pharmacogenetic issue, not a compliance issue — more frequent or more generous application will not overcome it
- Oral minoxidil bypasses the follicular bottleneck via hepatic activation, and performs at least as well in head-to-head data
- A genetic enzyme test exists, but for most patients, starting with oral minoxidil directly is simpler and achieves the same end
References
- Goren A, Castano JA, McCoy J, Bermudez F, Lotti T. Novel enzymatic assay predicts minoxidil response in the treatment of androgenetic alopecia. Dermatol Ther. 2014;27(6):171–173. doi:10.1111/dth.12111
- Gupta AK, Talukder M, Venkataraman M, Bamimore MA. Minoxidil: a comprehensive review. J Dermatolog Treat. 2022;33(4):1896–1906.
- Suchonwanit P, Thammarucha S, Leerunyakul K. Minoxidil and its use in hair disorders: a review. Drug Des Devel Ther. 2019;13:2777–2786.
- Penha MA, Shapiro J, Lartey RT, et al. Oral minoxidil versus topical minoxidil for male androgenetic alopecia: a randomized clinical trial. JAMA Dermatol. 2024;160(2):156–163.