Minoxidil is the most-used hair drug in the world, yet the foundational experiments were skipped. No one has measured its active form in blood, seen its enzyme in a human follicle, or tested whether the scalp deactivates it as fast as it turns it on.
Minoxidil is the most-used hair-loss treatment on earth. It has been sold for four decades, sits in millions of bathroom cabinets, and is the first thing almost every dermatologist reaches for. You would assume the science behind it is settled.
It is not. For a drug this common, the foundational experiments were quietly skipped. Nobody has measured its active form in human blood. Nobody has photographed its key enzyme in a human hair follicle. Nobody has checked whether your scalp switches the drug back off as fast as it switches it on. These are not obscure details. They are the questions that decide whether minoxidil works for you, and most of them have never been answered.
The short answer: Minoxidil is a prodrug. Your scalp has to convert it into minoxidil sulfate before it grows any hair, and that active form has never actually been measured in human blood, never been stained in a human follicle, and may be switched back off by scalp enzymes nobody counts. The cheap test that predicts your response has never been independently validated, and the gene everyone studies (SULT1A1) may matter less than the potassium channel the drug ultimately opens. Below are nine open questions, and why each one is still open.
We do not know. Minoxidil is a prodrug: the molecule you apply or swallow is inactive until an enzyme converts it into minoxidil sulfate, the form that actually stimulates the follicle (Buhl 1990). Every human blood measurement ever published quantifies the inactive parent drug, not the active sulfate (Fleishaker 1989).
The common assumption is that the sulfate is simply too unstable to measure. That is not supported: the chemistry work shows it is only slowly broken down in water, stable enough to isolate and study (McCall 1983). The real reason it has never been found in blood is that the body barely makes it systemically. The liver mostly attaches a glucuronide group instead (Gottlieb 1972), and in the one human sample anyone has ever fully profiled, the sulfate appeared in urine, and no one has ever even tried to measure it in blood.
That leaves a genuinely open question, and it matters most for oral minoxidil: if the active form is not circulating, then oral minoxidil, like topical, still has to be activated locally at the follicle. A sensitive modern assay could finally settle whether any active drug reaches the follicle through the bloodstream at all.
No. SULT1A1 is named in essentially every explanation of how minoxidil works, yet no published study has ever immunostained it inside a human hair follicle. The only protein-localization images come from rats (Dooley 1991), and a major human protein atlas lists SULT1A1 as "not detected" in skin.
The reason is technical, and it reshapes how the experiment has to be done. SULT1A1 is 90 percent or more identical to its close relatives SULT1A2 and SULT1A3, so standard antibodies cannot reliably tell them apart. A clean answer needs an RNA-based method that can distinguish the genes, not a naive antibody stain. Until that is done, the enzyme everyone invokes has, in a literal sense, never been seen in the tissue it is supposed to act in.
It has never been independently checked. The plucked-hair "minoxidil response test" measures sulfotransferase activity on a hair root using a cheap color-change reaction, and its accuracy figures come from a single research group (Goren 2014). No independent laboratory has ever validated it against a definitive method like mass spectrometry.
There is also a warning sign in the literature: one study found the test running in the opposite direction for oral minoxidil, where lower measured enzyme activity was associated with a better response, not a worse one (Jimenez-Cauhe 2024). A test that predicts one way for topical and the other way for oral is a test that is not yet understood. Re-running it with real analytical chemistry would either validate it as a genuine companion diagnostic or retire it.
Only in mice. The classic proof that minoxidil depends on sulfation, blocking the enzyme or its cofactor abolishes the drug's effect while the pre-formed sulfate still works, was done in mouse whisker follicles (Buhl 1990). It has never been repeated in human follicles.
When minoxidil was tested directly on human follicles in organ culture, it actually failed to grow them in a study of 36 donors (Magerl 2004), most likely because the experiment used the inactive parent drug and the follicles' own enzyme activity had faded in culture. Nobody has gone back and closed that loop with a clean, controlled human-follicle experiment. The single most-cited fact about minoxidil, that it needs SULT1A1, rests on mouse data.
Because it will not survive in the bottle. Minoxidil sulfate is roughly 14 times more potent than the parent drug you actually apply (Buhl 1990). In one small, uncontrolled study, a 10 percent minoxidil sulfate solution grew hair in almost every one of 44 patients who had already failed regular minoxidil (Chang 2017).
So the obvious product does not exist for a formulation reason, not a biology one: in a water- or alcohol-based vehicle, the sulfate slowly reverts to the inactive parent drug (McCall 1983), fine for a quick experiment but far too unstable for a product meant to sit on a shelf for years (Dias 2018). This is a different problem from the "unstable in blood" myth. In the body the issue is that little is made; in a bottle the issue is shelf stability. The active form of the world's most popular hair drug works. It is simply too unstable to sell in a conventional vehicle, which makes a stabilized formulation a real, unsolved problem rather than a closed door.
Probably the main one, but not the only one that can. In a test tube, at least four different sulfotransferases can sulfate minoxidil, including SULT1A1, SULT1A3, SULT2A1, and SULT1E1 (Anderson 1998). In the follicle, SULT1A1 is the dominant activator, and blocking sulfation does appear to shut the response down.
What nobody has tested is redundancy: whether a backup enzyme quietly covers when SULT1A1 is low or blocked. It matters, because if a second enzyme can pick up the slack, then a "low-SULT1A1" scalp is not automatically a lost cause, and blocking SULT1A1 with a supplement would not fully stop activation. A community scientist raised exactly this point. The honest answer is that no one has ever knocked out SULT1A1 in a human follicle and measured whether activation survives.
Barely, if at all, and this is the frustrating asymmetry at the center of the drug. SULT1A1 is easy to block and very hard to raise.
On the blocking side there is a real ladder of potent inhibitors that work at tiny concentrations: mefenamic acid, quercetin, curcumin, the EGCG in green tea, and the ethinylestradiol in combined birth control (Vietri 2000; Cook 2016; Rohn 2012). On the raising side there is almost nothing. SULT1A1 is a constitutive housekeeping enzyme that barely responds to induction. The one signal in human scalp is topical tretinoin, and even that only nudged the enzyme in people who started with low activity (Sharma 2019). Some products sold as "SULT1A1 boosters" target the wrong enzyme entirely: vitamin D, for instance, raises a different sulfotransferase, SULT2B1b, that does not act on minoxidil (Seo 2013).
Two honest caveats matter here. Most of these inhibitors are limited by whether they ever reach the scalp at meaningful levels, so "potent in a test tube" rarely means "matters on your head." And the birth-control link is an in-vitro enzyme finding only: the amount of ethinylestradiol circulating on the pill runs well below the level needed to inhibit the enzyme, so this is a shared-enzyme curiosity, not a reason to change your contraception.
The field has focused almost entirely on one enzyme gene, SULT1A1, where a single variant is loosely tied to response (Ramos 2021). But the more interesting genes may be at the other end of the pathway.
Minoxidil sulfate ultimately works by opening one specific potassium channel in the dermal papilla, built from the genes KCNJ8 and ABCC9 (Shorter 2008). Here is the striking part: people born with overactive versions of those exact genes have a condition called Cantu syndrome, whose hallmark is excessive hair growth all over the body (Harakalova 2012). In other words, minoxidil is essentially a drug that mimics Cantu syndrome for a few hours. Yet no one has checked whether common, everyday variants in those channel genes help decide who responds to the drug and who does not. The activation side of minoxidil genetics has been studied; the target side is almost untouched.
Possibly, and almost nobody is looking. The scalp does not only switch minoxidil on. Human hair follicle cells also make sulfatases, enzymes that strip the sulfate group back off and turn the active drug inactive again (Ichida 2020).
That opens a genuinely new way to think about non-response. A person who does not respond may not have too little of the activating enzyme. They may have too much of the deactivating one. No study has ever measured both the on-switch and the off-switch in the same follicle. The plucked-hair test reads only a net signal, so someone scored as "low SULT1A1" might in truth be high-deactivation, and would not be helped by any product designed to boost the activating enzyme. Measuring the balance, not just the on-switch, could redefine what a "non-responder" actually is.
If you want the practical version of this, our explainer on why minoxidil works for some people and not others covers the responder story, what blocks SULT1A1 covers the everyday inhibitors, and how minoxidil actually works covers the mechanism once the drug is switched on. Not sure where you fall? Take the hair quiz or see treatment options.
No. Every published human measurement quantifies the inactive parent drug, not the active sulfate (Fleishaker 1989). The reason is not that the sulfate is unstable, it is only slowly broken down in water (McCall 1983), but that the body barely makes it systemically. The liver mostly makes a glucuronide instead (Gottlieb 1972), and in the one fully profiled human sample the sulfate appeared in urine, and no one has ever even tried to measure it in blood.
Because it is too unstable in a conventional vehicle. Minoxidil sulfate is about 14 times more potent than the parent drug (Buhl 1990) and grew hair in a small uncontrolled study of patients who had failed regular minoxidil (Chang 2017), but in a water or alcohol base it slowly reverts to the inactive parent drug (McCall 1983), far too fast for a product with a multi-year shelf life (Dias 2018). It is a shelf-stability problem, not an efficacy problem.
It has never been independently validated. The accuracy figures come from a single group's cheap color-change assay (Goren 2014), no outside lab has confirmed it with mass spectrometry, and one study found it ran in the opposite direction for oral minoxidil (Jimenez-Cauhe 2024). Treat it as one input, not a verdict.
There is no evidence that it changes hair outcomes. The estrogen in combined birth control does inhibit the activating enzyme in a test tube (Rohn 2012), but the amount circulating on the pill runs well below the level needed to inhibit it in the body. It is a shared-enzyme curiosity, not a reason to change your contraception.
The standard explanation is that your scalp enzyme SULT1A1 has to convert minoxidil into its active form, and people vary in how much they have. But that is only one node. Delivery to the follicle, cofactor supply, how fast the drug is deactivated by scalp sulfatases, and the genetics of the target potassium channel may all matter, and most have never been measured together.
This article is for educational purposes only and is not medical advice. It describes open scientific questions and unproven hypotheses, which are labeled as such; nothing here should be read as a treatment recommendation. Oral minoxidil is used off-label for hair loss and should be undertaken only under the supervision of a licensed clinician; topical minoxidil is FDA-approved for that use. Always consult a qualified healthcare provider before starting, stopping, or changing any treatment.