Finasteride stops hair loss by blocking the hormone responsible for shrinking your hair follicles. It works as a competitive inhibitor of an enzyme called 5-alpha reductase, which normally converts testosterone into a more potent hormone called dihydrotestosterone, or DHT. By lowering DHT levels, finasteride slows follicle miniaturization and, in many men, allows thinning hair to recover.
The Role of DHT in Hair Loss
Testosterone itself isn’t the main driver of male pattern hair loss. The real culprit is DHT, a hormone your body produces when an enzyme called 5-alpha reductase acts on testosterone. DHT binds to receptors in genetically susceptible hair follicles, particularly on the top and front of the scalp, and gradually causes them to shrink. Over successive growth cycles, the affected follicles produce thinner, shorter, lighter hairs until they eventually stop producing visible hair altogether.
The 5-alpha reductase enzyme comes in several forms. The type II version is concentrated in the outer root sheaths of hair follicles, which is why it plays such a direct role in scalp hair loss. Finasteride specifically targets this type II isoenzyme (and to some extent the type III form), while having minimal effect on the type I version found elsewhere in the body. This selectivity is what makes it useful for hair loss at a relatively low dose.
How Finasteride Lowers DHT
Finasteride is a competitive inhibitor, meaning it occupies the same binding site on the enzyme that testosterone would normally use. With the enzyme blocked, less testosterone gets converted, and DHT levels drop in both your bloodstream and your scalp tissue. The standard 1 mg daily dose used for hair loss is enough to produce a meaningful reduction in DHT, though testosterone levels themselves remain largely unchanged.
This is an important distinction: finasteride doesn’t lower your testosterone. It simply prevents a portion of that testosterone from being converted into its more potent derivative. Your body still produces and uses testosterone normally for muscle function, energy, and other processes.
What Finasteride Does to Your Hair
With less DHT attacking your follicles, the miniaturization process slows or stops. Follicles that haven’t fully shut down can begin producing thicker, longer hairs again. A study published in the Journal of the American Academy of Dermatology found that men taking 1 mg of finasteride daily saw a net hair count increase of about 9% at 48 weeks and 15% at 96 weeks compared to placebo. Notably, hair weight increased even more than hair count alone, meaning individual hairs were growing thicker and longer, not just more numerous.
Finasteride works on both the crown and the frontal scalp. Early studies focused on vertex (crown) hair loss, but research specifically evaluating men with anterior and mid-scalp thinning found significant improvements in hair count and appearance there as well. That said, results tend to be most dramatic in areas where follicles are still partially active. If a region of the scalp has been completely bald for years, those follicles may be too far gone for finasteride to revive.
How Long Results Take
Finasteride is not a fast fix. The hair growth cycle is slow, and lowering DHT doesn’t produce overnight changes. Here’s what the typical timeline looks like:
- Month 1: No visible changes. DHT levels are dropping, but hair follicles haven’t had time to respond yet.
- Months 3 to 6: Many men notice reduced shedding first, followed by subtle regrowth. Some experience a brief increase in shedding early on as weaker hairs are pushed out by new growth.
- Months 6 to 12: Regrowth and thickening become more visible. Hair count improvements continue building.
- Month 12: Clinical studies show this is when maximum hair count increases typically occur. Dermatologists generally evaluate whether finasteride is working at the one-year mark.
Some men continue to see gradual improvements beyond the first year. A 10-year follow-up study of 118 men found that 21% of participants who continued treatment past five years achieved better results than they had at the five-year mark. The drug’s effectiveness didn’t diminish over time, and a large proportion of men who appeared to plateau at one year improved further with continued use.
What Happens If You Stop
Finasteride only works while you’re taking it. Once you stop, DHT levels return to their previous baseline, and the follicle miniaturization process resumes. Hair gained during treatment is gradually lost, and within about 6 to 12 months of stopping, most men return to the level of hair loss they would have had without treatment. This means finasteride is a long-term commitment if you want to maintain the results.
Side Effects and Their Frequency
The most commonly discussed side effects are sexual in nature. In clinical trials, men taking finasteride for hair loss at 1 mg daily reported slightly higher rates of reduced sex drive, erectile difficulty, and changes in ejaculation compared to men taking a placebo. A review of multiple studies found statistically significant increases in sexual side effects among men with hair loss treated with 1 mg finasteride daily versus placebo, though the absolute percentages were relatively small.
For context, studies using the higher 5 mg dose (prescribed for prostate conditions, not hair loss) found erectile dysfunction in roughly 3% to 16% of patients compared to 2% to 6% on placebo, and decreased libido in about 2% to 10% versus 1% to 6% on placebo. The 1 mg dose used for hair loss carries lower rates, but the gap between finasteride and placebo is real. In the 10-year follow-up study, 6% of men reported side effects, though some of those chose to continue treatment because they were satisfied with their hair results.
Most men who experience sexual side effects find they resolve after stopping the medication. A small number of men report persistent symptoms, a topic that remains under investigation and debate in the medical literature.
Safety for Women and Household Members
Finasteride is not approved for use in women, and it poses a specific risk during pregnancy. Because it blocks the conversion of testosterone to DHT, it can cause developmental abnormalities in a male fetus. The FDA has warned that women should not handle crushed or broken finasteride tablets because the drug can be absorbed through the skin. Intact tablets have a coating that prevents contact with the active ingredient, but compounded topical formulations typically lack this barrier. If you use a topical finasteride product, be aware of the potential for skin-to-skin transfer to female partners or household members.