Finasteride: The Drug That Fixes Your Hair by Annoying an Ex You Can't Fully Get Rid Of

5-alpha reductase. It's an enzyme. What does it do, you might ask? Well — it converts free testosterone into a different version called dihydrotestosterone, more commonly known as DHT.

Now, DHT is like that ex you were madly in love with but eventually lost feelings for. She cared for you, helped you grow, and played a genuinely meaningful part in your life — but at some point you just grew past it and stopped feeling anything. Does that make you an asshole? Maybe. But here's the thing: this ex keeps hanging around. She comes to the same parties as you, still wishes you on your birthday, still texts you goodnight. You have no clean way to get rid of her.

Now, how is DHT like this ex, you ask? DHT plays a genuinely pivotal role during puberty. It grows hair in places that were previously bare, deepens your voice from something resembling a chipmunk to something resembling Elvis Presley, and makes your little Johnny grow a little taller and a little longer. Genuinely important stuff. But once you've gone through all of that and finished growing, science hasn't found DHT a clear job to do anymore. That doesn't mean production stops, though — it still hangs around, floating aimlessly through your bloodstream like an ex who never got the group chat memo that things ended. And at some point, DHT gets bored, and starts messing with two organs in particular: the prostate gland, causing it to enlarge (benign prostatic hyperplasia, or BPH), and your hair, causing it to fall out (androgenetic alopecia — andro = hormonal, genetic = you get the gist, alopecia = hair loss).

One of the ways you treat BPH is by blocking DHT production in the body. How? You block the conversion of free testosterone into DHT, by inhibiting the enzyme responsible for that conversion — 5-alpha reductase. This gives you a class of drugs called 5-alpha reductase inhibitors. Makes sense so far.

Finasteride belongs to this class of 5AR inhibitors — and here's a fact worth checking, so I checked it: doctors administering finasteride to BPH patients in the early '90s did observe increased hair growth as a side effect. Merck had originally developed and released finasteride at 5mg under the brand name Proscar, in 1992, purely for enlarged prostates. The hair growth observation was compelling enough that Merck pursued a second approval specifically for hair loss, at a much lower 1mg dose, under the name Propecia — approved by the FDA in December 1997, becoming the first oral drug in history proven to treat male pattern baldness in the majority of men who use it. Not a marketing legend. Documented history.

Oral dosing for AGA: The standard dose for hair loss is 1mg daily, taken with or without food — a fifth of the 5mg BPH dose, since the hair loss job doesn't need nearly as much DHT suppression as the prostate job does. You typically need three months or more of consistent daily use before you see any benefit, since it's working on a hair growth cycle, not a light switch. Whatever benefit you get, you keep only by continuing — stop, and the hair you gained tends to drift back toward the trajectory it was on before you started. There's no loading dose, no cycling on and off, and no evidence that going above 1mg does anything extra for hair specifically, even though it clearly does more for the prostate at 5mg.

Side effects: The headline concern is sexual — reduced libido, difficulty achieving or maintaining an erection, and a decrease in ejaculate volume, reported by a minority of users. Less commonly, some men report breast tenderness or mild enlargement. The reassuring part, which gets buried under the scarier part, is that in the large controlled trials most of these effects resolved either on their own during continued use or after stopping the drug. It's not nothing, and it deserves to be taken seriously rather than dismissed — but it's also not the coin-flip Reddit sometimes makes it sound like.

Why men get scared for their little Johnny specifically: because it's the one side effect that's both highly visible to the person experiencing it and highly embarrassing to discuss with anyone else — which is exactly the combination that makes a story travel. A guy who had zero issues on finasteride has no reason to ever post about it. A guy who did have an issue, especially a rare persistent one, has every reason to post about it loudly and often — and a small, vocal, long-running online community (sometimes discussed under the term "post-finasteride syndrome," which remains genuinely debated and unresolved in the medical literature) ends up looking, from the outside, far larger and more representative than it statistically is. That's availability bias again — the same mechanism from the minoxidil article, just aimed at a more sensitive organ this time.

The actual statistics: in the pivotal clinical trials that led to Propecia's approval, sexual side effects were reported in roughly 3.8% of men on finasteride versus 2.1% on placebo in the first year — a real difference, but a small one, and one where the placebo group wasn't at zero either, which tells you some of what gets blamed on the drug isn't purely the drug. Longer-term extension studies out to five years found no evidence that the incidence of these side effects increased the longer men stayed on it — if anything, new reports became less frequent over time, not more.

Topical finasteride: if the systemic exposure still worries you even at those odds, topical finasteride exists specifically to address that. Controlled trials of a topical finasteride solution have shown improvements in hair count comparable to the oral 1mg tablet, while limiting how much of the drug actually enters general circulation — less of the drug traveling to organs that were never the target in the first place. It's not automatically zero-risk, but it's a genuinely different risk profile worth discussing with your doctor rather than dismissing outright.

Now, taking fin can be scary, and there are real risks involved with side effects. This is not an article telling you that you must, mandatorily, get on finasteride to save your hair. It's simply information to help you make an informed decision, because almost nobody says it plainly. Some people underplay the possible side effects, some doctors overplay them, and in reality, most doctors aren't especially well-read on this specific drug class in connection to hair loss — the side effect profiles they recall are usually from the higher BPH dosing regime, which they then mentally apply, incorrectly, to the much lower dose actually used for hair loss.

Anyway — if you do decide to get on fin, always consult with a doctor first. You can get that done with us at ADAM as well.

Scientific Citations

1. Finasteride's development history and FDA approval timeline. Confirms finasteride was originally developed and approved (as Proscar, 5mg) for benign prostatic hyperplasia in 1992, with hair growth observed as a side effect during BPH trials, leading to the 1mg dose (Propecia) being separately approved for androgenetic alopecia in December 1997. https://www.americanhairloss.org/hair-loss-treatment/drug-therapy/finasteride/ ; https://www.drugs.com/finasteride.html
2. PROPECIA (finasteride) Tablets, 1 mg — Prescribing Information. U.S. FDA. Official label data on dosing (1mg/day), onset of benefit (3+ months), reversibility of benefit upon discontinuation, and reported sexual adverse effects (reduced libido, erectile difficulty, decreased ejaculate volume). https://www.accessdata.fda.gov/drugsatfda_docs/label/2011/020788s018lbl.pdf
3. Kaufman KD. Use of Finasteride in the Treatment of Men With Androgenetic Alopecia (Male Pattern Hair Loss). Summarizes peer-reviewed data including long-term (5-year) placebo-controlled trial results, showing sexual side effect incidence did not increase with duration of use. https://www.sciencedirect.com/science/article/pii/S0022202X15529357
4. Finasteride topical solution vs. oral tablet — randomized, double-blind, double-dummy, parallel-group 24-week study. Found a 0.25% w/w topical finasteride solution produced hair count improvements comparable to the oral 1mg tablet. Referenced via https://www.drugs.com/finasteride.html