- "DHT blocker" is a marketing label, not a drug class. It covers at least four different kinds of compound with different mechanisms, evidence, and legal status.
- Finasteride and dutasteride reduce DHT production. They are licensed medicines with the strongest evidence in the category.
- Ketoconazole shampoo has modest adjunct evidence. Saw palmetto and other naturals have weak, mixed evidence.
- RU-58841 and pyrilutamide block the androgen receptor rather than lowering DHT. They are unlicensed research compounds, sold for laboratory use only.
What DHT is, and why everything targets it
Dihydrotestosterone, DHT, is a potent androgen made from testosterone by the enzyme 5-alpha-reductase. In scalp follicles that are genetically sensitive to it, DHT binding at the androgen receptor drives progressive miniaturisation: each growth cycle produces a finer, shorter hair, until the follicle stops producing visible hair at all. That is pattern hair loss, and it is why nearly every compound in this space, licensed or not, targets the DHT pathway somewhere.
The pathway has two obvious points of attack. You can reduce how much DHT gets made, by inhibiting 5-alpha-reductase. Or you can block what DHT binds to, by occupying the androgen receptor. Everything sold as a DHT blocker does one of these two things, does it weakly, or does not really do either. Sorting the category by mechanism, rather than by marketing, is the whole point of this guide.
Category one: 5-alpha-reductase inhibitors
These are the production reducers, and they are what most guys actually mean when they say DHT blocker. They are also the only part of the category with strong clinical evidence.
Finasteride
Finasteride inhibits the type II isoform of 5-alpha-reductase, the form that dominates in scalp follicles and the prostate. At the 1 mg oral dose it reduces serum DHT by roughly 70 percent. It is a licensed medicine in the UK, prescription only, with decades of trial and post-marketing data behind it. Its efficacy for pattern hair loss is well established in that literature, and so is its documented side effect profile, both of which are matters for the prescribing information and a clinician rather than this site. As a reference point for everything else in this guide, finasteride is the benchmark: any compound claiming the DHT blocker label is implicitly being compared to it. We look at how the research antiandrogens differ from it in RU-58841 vs finasteride.
Dutasteride
Dutasteride inhibits both type I and type II 5-alpha-reductase, which takes serum DHT suppression above 90 percent. It is licensed in the UK for benign prostatic hyperplasia rather than hair loss, though it is licensed for hair loss in some other countries, South Korea and Japan among them, and UK prescribers can use it off-label at their discretion. Mechanistically it is finasteride with broader enzyme coverage and a much longer half-life. It is the most complete DHT suppression available in a licensed medicine, and like finasteride it is systemic: it lowers DHT everywhere, not just in the scalp.
Topical finasteride and topical dutasteride
The same molecules also exist in topical form, with the aim of concentrating the effect in the scalp while reducing systemic exposure. The compound and mechanism are unchanged, only the delivery route differs, and some systemic absorption still occurs. Topical finasteride has genuine trial data behind it and is available via prescription in the UK through specialist routes. Because the topical corner of this category is its own subject, with its own two-mechanism distinction, we cover it separately in our guide to topical DHT blockers rather than repeating it here.
Category two: androgen receptor antagonists
The second real mechanism in the category takes the opposite approach. Instead of lowering DHT, a receptor antagonist competes with DHT for the androgen receptor itself. DHT levels stay exactly where they were; what changes is whether DHT can dock and signal. Strictly speaking these compounds do not block DHT at all, they block its receptor, but the label has stuck to them anyway.
The two compounds most associated with this approach in hair research are RU-58841 and pyrilutamide (KX-826). Both are non-steroidal androgen receptor antagonists designed for topical application, with the intent of acting locally at the follicle rather than across the body. Neither is a licensed medicine anywhere. RU-58841 was developed by Roussel-Uclaf in the 1980s and abandoned before completing clinical development, so its human evidence base is thin, a question we examine honestly in does RU-58841 actually work. Pyrilutamide went further, through Phase 3 trials in China, with mixed topline results and no marketing approval to date. Full profiles of each are in what is RU-58841 and what is pyrilutamide, and the two are compared directly in RU-58841 vs pyrilutamide.
The theoretical appeal of the receptor approach is that it leaves systemic DHT untouched, which matters because DHT does normal physiological work outside the scalp. The honest counterweight is evidence: the 5-alpha-reductase inhibitors have large completed trial programmes, and the receptor antagonists do not. That gap, not mechanism, is why one group consists of licensed medicines and the other consists of research compounds. In the UK, compounds like RU-58841 are legal to buy and possess for research purposes but are not approved for human use, a status we unpack in is RU-58841 legal in the UK.
Category three: ketoconazole shampoo
Ketoconazole is an antifungal, and its 2% shampoo form is a UK pharmacy medicine for seborrhoeic dermatitis and dandruff. It earns its place in DHT blocker lists because it has shown mild antiandrogenic activity in laboratory work and because a small number of studies, most cited among them a 1998 comparative study, found modest improvements in hair density with regular use. The evidence is real but limited: small studies, short durations, and effect sizes that position it as an adjunct rather than a primary intervention. Part of its effect may also come from reducing scalp inflammation and Malassezia load rather than from androgen signalling at all. It is best understood as a supporting act with pharmacy-medicine convenience, not as a serious standalone rival to the compounds above.
Category four: natural and supplement DHT blockers
This is the largest category by product count and the weakest by evidence, and honesty matters more here than anywhere else in the guide.
Saw palmetto
Saw palmetto extract inhibits 5-alpha-reductase in vitro, which gives it a plausible mechanism on paper. The human data is another matter: the studies are small, methodology varies widely, results are mixed, and where benefit appears it is clearly weaker than finasteride's. A frequently cited comparative study found finasteride outperformed saw palmetto convincingly over two years. It is not nothing, but it is a long way from established.
The rest of the supplement aisle
Pumpkin seed oil has one small placebo-controlled trial with methodological questions. Green tea extract, zinc, biotin, rosemary oil, and stinging nettle all appear on DHT blocker lists with mechanistic hand-waving and little or no controlled human data for pattern hair loss. Biotin in particular has essentially no evidence for androgenic hair loss in the absence of deficiency. None of this means every supplement is useless. It means that if the question is what the evidence supports, the supplement category mostly cannot answer it, and products in this space are often marketed with a confidence the data does not justify.
How to read the evidence in this category
The DHT blocker label flattens an enormous evidence gradient, so it helps to have a simple hierarchy in mind. At the top sit the licensed medicines, finasteride and dutasteride, with large randomised controlled trials, regulatory review, and decades of post-marketing surveillance. One step down is ketoconazole shampoo, with small controlled studies and a plausible adjunct role. Below that sit the research antiandrogens, which have real mechanistic and preclinical work, and in pyrilutamide's case substantial clinical trials, but no completed pathway to approval. At the bottom is the supplement category, where the distance between marketing claims and controlled human data is widest.
Two habits make the category much easier to navigate. First, always ask which mechanism a product actually has: production reduction, receptor blockade, or neither. A shampoo listing "DHT blocking botanicals" is making a different kind of claim from a prescription for finasteride, even though both might use the same phrase. Second, ask what kind of study supports the claim. In vitro enzyme inhibition is a starting point, not a result. A compound can inhibit 5-alpha-reductase in a dish and do nothing measurable on a human scalp, because concentration, delivery, and duration all have to line up. Most of the weaker entries in this category fail at exactly that step.
Systemic versus local, the other axis that matters
Mechanism is one axis of the map. The other is where a compound acts. Oral finasteride and dutasteride are systemic by design: they lower DHT throughout the body, which is what produces both their efficacy data and the side effect discussions that surround them. The topical route, whether applied finasteride or a receptor antagonist like RU-58841, aims to confine the effect to the scalp. Whether any topical compound truly stays local is a pharmacokinetics question that has been studied properly for the licensed molecules and only partially for the research ones. That distinction, local intent versus proven local behaviour, is one of the most important open questions in the research antiandrogen literature, and it is a large part of why these compounds remain research materials.
The full map, side by side
- Oral finasteride: type II 5-alpha-reductase inhibitor, ~70% serum DHT reduction, licensed, strongest evidence in the category.
- Oral dutasteride: dual type I and II inhibitor, 90%+ DHT reduction, licensed medicine, off-label for hair in the UK.
- Topical finasteride and dutasteride: same molecules, local delivery, covered in the topical guide.
- RU-58841 and pyrilutamide: topical androgen receptor antagonists, unlicensed, limited human data, research use only.
- Ketoconazole 2% shampoo: pharmacy medicine, mild adjunct evidence.
- Saw palmetto and other naturals: plausible mechanisms, weak and mixed human evidence.
Where Norwood Labs sits in this landscape
We occupy one clearly defined corner of the map: the research antiandrogens. Norwood Labs supplies RU-58841 and pyrilutamide (KX-826) as batch-assayed reference materials for laboratory research, with a published certificate of analysis for every batch. We do not sell finasteride, dutasteride, ketoconazole, or supplements, we do not publish usage or application instructions, and nothing on this site is medical or treatment advice. If you are considering a licensed medicine, that conversation belongs with a GP or pharmacist. If you are researching the receptor antagonists, our UK buying guide covers what to verify before ordering from any supplier, and the RU-58841 side effects article summarises what the research literature does and does not establish.