RU58841 is a topical anti-androgen used for male and female pattern hair loss. You apply it as a solution to the scalp once a day and it sits on the androgen receptors in your hair follicles, blocking DHT from binding and triggering the miniaturisation process that thins hair over time. Unlike finasteride, it doesn't lower DHT anywhere else in your body, the whole point of the molecule is that it gets used up locally and barely makes it into systemic circulation.

It get the scalp-level effect of an anti-androgen without the systemic sexual, mood, or hormonal side effects that Finasteride and Dutasteride can cause in a meaningful minority of users. It's the option people reach for when they want to keep their hair but don't want to gamble with their libido, mental clarity, or fertility. It's also the option women with pattern hair loss reach for when oral anti-androgens are off the table due to side effects, pregnancy plans, or contraception conflicts.

It was developed by Roussel Uclaf in France in the early 1990s, advanced through Phase II human trials by ProStrakan in the 2000s with promising results, then quietly discontinued for non-safety business reasons. No company has picked it up since. That's why it sits in the research-chemical bucket today, sold as raw powder you mix into a solution yourself or as pre-mixed bottles from grey-market vendors of variable quality.

Deep-dive

Dosage:

• Standard concentration: 5% solution, applied topically to the scalp once daily. This is the concentration used in the macaque and human studies and the one with the most data behind it. Lower concentrations (3%, 2.5%) work but the dose-response data suggests 5% is where the effect plateaus
• Daily dose: 50 mg of RU58841 per day for men (1 mL of a 5% solution). Women with pattern hair loss can start at 25-50 mg/day, depending on the area being treated and skin sensitivity. There's no benefit to going higher than 5%, the receptor occupancy plateaus and you just irritate the scalp more
• Vehicle: 70% ethanol + 30% propylene glycol is the standard mix and the one used in the original studies. Ethanol drives penetration, PG keeps the drug in solution and slows evaporation. If you're sensitive to PG (some people get contact dermatitis), the alternative is K&B solution (a PG-free oil-based vehicle) but absorption is slightly lower. Pre-mixed solutions from research-chemical vendors save the hassle of weighing powder but quality varies, prefer vendors who publish third-party HPLC purity testing
• Application: Apply to a clean, dry scalp, part the hair and target the actual scalp surface rather than the hair shafts. Anything that lands on hair won't absorb. Most people use 1 mL spread across the affected zones (typically the frontal third and crown). Let it dry fully (10-15 minutes) before lying down or applying anything else. Wash hands after application
• Timing: Once daily is enough. The original twice-daily protocol in the ProStrakan trials hasn't shown a clear advantage over once daily and doubles the irritation risk. If stacking with topical minoxidil, separate the two by 10-15 minutes so each has time to absorb. Most people apply RU in the morning and minoxidil at night
• Stability: RU58841 in ethanol/PG solution is reasonably stable at room temperature for a few months but degrades faster in heat and light. Keep your bottle in a cool, dark place. Make small batches if you're mixing your own powder, 1-2 months of solution at a time, rather than a year's worth
• Cycling: Unlike some peptides, RU doesn't need to be cycled. The receptor antagonism doesn't downregulate the receptor itself. Stopping leads to hair loss returning within roughly 3 months as the follicles are no longer protected

Here's what you can expect:

The first 4-8 weeks usually show very little visible change. Some users notice an initial shed (similar to the minoxidil shed) as follicles shift from telogen back into anagen, this is a positive sign even though it looks alarming. By month 3, most users see thicker existing hair and slowing of further loss. By month 6, regrowth in previously thinned areas becomes visible in photos taken under consistent lighting. The macaque data showed peak effect somewhere between month 5 and month 7 and continued maintenance with ongoing treatment.

It's not a one-time fix. The day you stop applying it, the protection ends and the underlying genetics take over again. Most users who get good results from RU58841 commit to it long-term, the same way finasteride users do. If you're not prepared to use a topical compound for years, the cost-benefit doesn't favour this one.

If you see no improvement by month 6 of consistent daily use, you're unlikely to be a responder and continuing is mostly burning money. The non-responder rate is probably similar to finasteride's (around 10-20% of users), though we don't have clean trial data on this for RU specifically.

Side effects & risks: