Finasteride and DHT: How Propecia Slows Male Pattern Hair Loss

When finasteride arrived as a hair-loss pill in 1997, it changed the conversation on every message board I was reading. Until then, the only approved option was minoxidil, a lotion that encourages follicles to grow. Finasteride did something different. It went after the hormone that drives male pattern baldness in the first place. Nearly thirty years later it is still one of only two drugs the FDA has approved for the condition, and it is still widely misunderstood.

What DHT does to a hair follicle

Male pattern hair loss, or androgenetic alopecia, is driven largely by dihydrotestosterone, usually shortened to DHT. Your body makes DHT from testosterone using an enzyme called 5-alpha reductase. In men who are genetically prone to balding, follicles on the crown and hairline are sensitive to DHT. Over years, exposure shrinks those follicles. Each new hair comes in a little thinner and shorter, until some follicles produce only fine, nearly invisible fuzz.

That gradual shrinking is why hair loss usually looks like thinning before it looks like baldness. It is also why timing matters so much. A follicle that is shrinking can often be rescued. One that has been dormant for many years is much harder to bring back.

How finasteride works

Finasteride blocks the type II form of 5-alpha reductase, the version most associated with the scalp and prostate. Less enzyme activity means less testosterone gets converted to DHT. According to the prescribing information, a single 1 mg dose lowers blood DHT levels by about 65% within 24 hours, and the drug lowers DHT in the scalp as well.

The drug was not designed for hair at all. Merck first developed it at 5 mg a day to treat an enlarged prostate. The lower 1 mg dose was later studied for hair loss and approved under the brand name Propecia.

What the trials showed

The main evidence comes from studies Merck ran in the mid-1990s, published in the Journal of the American Academy of Dermatology in 1998. The trials enrolled 1,879 men aged 18 to 41 with mild to moderate hair loss and compared 1 mg of finasteride a day with placebo.

• At one year, men on finasteride had noticeably more hair on the crown than men on placebo, measured by counting hairs in a one-inch circle.
• At five years, investigators judged 77% of the men on finasteride to have more hair, compared with 15% of the placebo group.
• Stopping progression may be the most important result. Over five years, 35% of treated men showed further hair loss, while every man in the placebo group did.

That last point is the one I emphasize when people ask me about finasteride. Its biggest strength is slowing or stopping loss. Some regrowth is a bonus, not the main event.

What to expect in practice

• Patience. The label says daily use for at least three months is usually needed before any benefit shows. A full year is a fairer test.
• It only works while you take it. When men stop, the gains are generally lost within about twelve months.
• It pairs well with minoxidil. The two work by different mechanisms. Finasteride addresses the hormone; minoxidil supports the follicle's growth phase. Many dermatologists use them together.
• It is a prescription drug. Finasteride requires a prescription in the US. A doctor should decide whether it is right for you.

Side effects and cautions

In the one-year trials, sexual side effects such as lower libido, erectile problems and changes in ejaculation were reported by 3.8% of men on finasteride, compared with 2.1% on placebo. Those rates fell over time, and most cases resolved, including in men who stopped the drug. Some men report problems that persist after stopping, which is something to discuss openly with your doctor.

In 2022 the FDA required that depression and suicidal thoughts reported after approval be listed in the Propecia label. Because those reports are voluntary, how often they occur cannot be reliably estimated. Anyone who notices mood changes while taking finasteride should contact their doctor promptly.

A few other points are worth knowing:

• Pregnancy. Finasteride must not be used by women who are or may become pregnant, because it can affect the development of a male fetus. Women who are or could be pregnant should not handle crushed or broken tablets.
• PSA tests. Finasteride lowers PSA readings, a blood test used to screen for prostate cancer. Tell your doctor you take it so results can be interpreted correctly.
• Not approved for women. The 1 mg dose is approved only for men, and a study in postmenopausal women found no benefit.

Where finasteride fits

For a man in the early or middle stages of pattern hair loss, finasteride is one of the best-studied tools available. It will not restore a fully bald scalp, and nothing about it is guaranteed. What it can do, for many men, is hold the line. Combined with a well-applied topical routine, such as tretinoin followed by minoxidil the way Dr. Lewenberg taught, it gives the follicles you still have the best chance of staying in the game. Talk with a physician about your history and goals before starting.