Where finasteride came from: the guevedoces
Finasteride grew out of a 1970s study of boys born without DHT in the Dominican Republic. Lowering DHT has costs too, including PFS after stopping.
Taking finasteride (Propecia) for hair loss? It began with a study of children in the Dominican Republic who were born unable to make a hormone called DHT.
Lowering DHT can also bring side effects that, for some people, last after stopping. That is called post-finasteride syndrome (PFS). The UK Propecia leaflet lists “persistent difficulty having an erection after discontinuation of treatment” among its side effects.
So who were these children, and how did they lead to a hair-loss pill?
Who were the guevedoces?
The guevedoces were children in a few villages in the Dominican Republic who were born looking like girls but were genetically male. The local name means, roughly, “penis at 12”.
In 1974, Dr Julianne Imperato-McGinley described them in the journal Science. They lacked an enzyme, 5-alpha reductase, which turns testosterone into a stronger hormone called dihydrotestosterone, or DHT.
Most were raised as girls. At puberty, the rise in testosterone did much of the work DHT had not. Their voices deepened, their muscles grew and they developed male genitals.
How did DHT lead to a hair-loss pill?
Researchers noticed what the grown men did not get. A review of the history by urologist Leonard Marks puts it plainly: “no acne occurs, the temporal hair line does not recede, and the prostate remains small”.
That pointed to DHT as a driver of two common problems in older men: an enlarged prostate and a receding hairline. If a pill could block the same enzyme, perhaps it could copy the effect in adults.
The same men also showed what DHT does in the body. Without it, their genitals had not developed normally before puberty. DHT does more than grow hair, so blocking it was always going to reach beyond the scalp.
Merck began a development programme at its laboratories in Rahway, New Jersey, and made finasteride, which blocks 5-alpha reductase. The US Food and Drug Administration approved it for an enlarged prostate on 19 June 1992, sold as Proscar at 5 mg.
A lower dose for hair loss in men followed. Propecia, finasteride 1 mg, was approved in the US in 1997.
The pill was built to lower DHT in grown men. What that does for each person, in the brain as well as the body, is still being studied.
Can the side effects last after stopping?
Yes, according to the UK leaflet. It lists erection problems, lower sex drive and problems with ejaculation that persisted after treatment was stopped, under “Frequency unknown”.
PFS is more than sexual problems. The 2022 diagnostic criteria say problems with cognition, depression and suicidality can occur even without sexual difficulties.
The same leaflet says to stop taking Propecia and contact your doctor if you get depression or suicidal thoughts. Nobody has measured how often lasting problems happen.
Sam took finasteride for hair loss. In this interview he describes what he was told before he started. Watch for the warning he remembers.
He was told erection problems might happen. Was anything said about them lasting? “Nothing permanent,” he says.
What can I do if I’m worried?
These medicines help many people. If you have side effects, that’s a decision for you and your prescriber. The leaflet says to stop and contact your doctor if your mood changes.
Three questions for your GP
- Could finasteride explain what I've noticed?
- What else should we rule out?
- What happens next if it doesn't ease after stopping?
You could start with: "I took finasteride 1 mg from March 2023 to January 2024, and I've had changes since."
Bring the leaflet line. Many people say it took time to be taken seriously, and you can report it without a doctor agreeing.
If you’re struggling or have thoughts of suicide, call Samaritans on 116 123, any time.
Report it: it helps everyone
A doctor’s note helps you. A report to the medicines regulator helps everyone else: it is how patterns get noticed.
- No proof needed. A suspicion is enough, even years after stopping.
- Where: the Yellow Card scheme in the UK, MedWatch in the US.
- Use the right words. If symptoms continued after stopping, write Post 5-alpha-reductase inhibitor syndrome (MedDRA 10082430).
Our reporting guide walks you through it
What else do people ask?
Is finasteride the same as Proscar? Yes, the medicine is the same. Proscar is the 5 mg tablet for an enlarged prostate, and Propecia is the 1 mg tablet for hair loss. Our page on Proscar side effects covers the higher dose.
Do the guevedoces lose their hair? The history review says their hairline does not recede and their prostate stays small. That observation is what made researchers look for a drug that blocks the same enzyme.
Does finasteride affect fertility or mood? The leaflet lists depression and suicidal thoughts. Read more on Propecia and male fertility and Propecia, depression and suicide.
Sam was told nothing would be permanent. If that sounds familiar, our PFS page is the place to start.
Sources
- Marks LS. 5α-reductase: history and clinical importance. Rev Urol 2004;6(Suppl 9):S11-S21. Children “with apparent female genitalia at birth” were raised as girls until puberty. In adult men “no acne occurs, the temporal hair line does not recede, and the prostate remains small”. Merck “began an ambitious development program in the research laboratories at Rahway, New Jersey”. The FDA “approved finasteride June 19, 1992”, and “FDA approval for male pattern hair loss (in men only) followed”. Reference 1: Imperato-McGinley J and colleagues, Science, 1974.
- Organon. Propecia 1 mg patient information leaflet, electronic Medicines Compendium, revised May 2026. “Frequency unknown”: “persistent difficulty having an erection after discontinuation of treatment”, “persistent decrease in sex drive after discontinuation of treatment”, “persistent problems with ejaculation after discontinuation of treatment”. Stop and talk to your doctor if you get depression or suicidal thoughts.
- Healy D, Bahrick A, Bak M and colleagues. Diagnostic criteria for enduring sexual dysfunction after treatment with antidepressants, finasteride and isotretinoin. Int J Risk Saf Med 2022;33(1):65-76. Proposed criteria from expert agreement. Section 5 (PFS): cognitive impairment, depression and suicidality “can occur independently of any sexual difficulties”.
- US Food and Drug Administration, Questions and answers: finasteride label changes, 11 April 2012 (archived; no longer on fda.gov). Propecia was approved in 1997.
- GOV.UK. Report a problem with a medicine or medical device.
- Moral Medicine, The Hidden Devastation of Finasteride: Sam’s Story, June 2025.
Sources checked on 22 October 2026. AI-assisted writing and review, not clinical review.
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