The short answer: finasteride works by lowering a hormone called dihydrotestosterone, or DHT, which is the main driver of male pattern hair loss. With less DHT reaching the follicles, the gradual shrinking that thins your hair slows down, and for many people some hair recovers. Here is how that happens, what the evidence shows, and how long it takes.
What is finasteride?
Finasteride is a prescription-only medicine, described by the NHS as a treatment for male pattern hair loss at the 1mg daily dose, and for benign prostate enlargement at a higher dose. In the UK it is not available for hair loss on the NHS, so it is prescribed privately after an online or in-person assessment. The hair-loss dose has been licensed since the late 1990s, so the evidence base behind it is long-running rather than new.
The DHT problem
Male pattern hair loss is largely about how sensitive your hair follicles are to DHT, a hormone made from testosterone. In genetically susceptible follicles, DHT causes them to shrink over successive growth cycles, producing finer, shorter hairs until they stop making visible hair at all. We explain this in more detail in what DHT does.
DHT is produced when an enzyme called 5-alpha-reductase converts testosterone into it. Around 5 to 10 per cent of circulating testosterone is converted this way, and scalp follicles in the classic pattern-loss zones carry more of the enzyme than follicles at the back and sides, which is why those areas thin first while the back usually holds. That conversion step is the point where finasteride intervenes.
What finasteride actually does
Finasteride blocks the type II form of 5-alpha-reductase. By inhibiting that enzyme, it reduces the amount of testosterone converted into DHT, so scalp DHT levels fall substantially: the UK product information reports scalp DHT falling by well over half on the 1mg daily dose. Less DHT means less pressure on the vulnerable follicles, which slows the miniaturisation process and, in many men, allows shrunken follicles to recover a little.
It is worth being clear about what this is not: finasteride does not add hair that was never there, and it does not stop the underlying genetic tendency. It changes the hormonal environment your follicles sit in, and that is enough to change the trajectory for most people who take it.
Does finasteride work? What the evidence shows
The pivotal trials of 1mg finasteride followed men with mild to moderate vertex hair loss for up to five years. In those studies, the large majority of men taking finasteride either kept the hair they had or showed measurable regrowth on standardised photographs, while most men on placebo continued to lose hair over the same period. The effect is strongest at the crown and mid-scalp and more modest at the temples, which is why where you sit on the Norwood scale shapes realistic expectations. What the first months actually feel like, including the shedding wobble some people get, is covered in our guide to the first year on finasteride.
Tablet, not serum
Finasteride is taken as a daily oral tablet. It works through the bloodstream to lower DHT body-wide, which is different from a topical scalp treatment such as minoxidil, which is applied where you want the effect. Because they work in completely different ways, the two are often used together. Topical finasteride exists but is not a licensed medicine in the UK, and the long-term evidence behind it is much thinner than for the tablet.
What matters is taking it consistently, at roughly the same time each day, so DHT stays suppressed rather than rebounding between doses. There is no benefit to taking more than the prescribed dose; a higher dose does not lower scalp DHT meaningfully further and simply raises the chance of side effects.
How long does finasteride take to work?
Finasteride is not fast. Because hair grows in slow cycles, the first sign of benefit is usually less shedding rather than obvious regrowth. Most people need three to six months before they notice a change, and around twelve months for a fair assessment of how much it is helping. Standardised photos taken at the start are genuinely useful, because week-to-week memory is unreliable.
A rough timeline looks like this. Months one to three: no visible change, DHT is suppressed within days but follicles respond slowly, and a temporary increase in shedding can happen as follicles reset their cycles. Months three to six: shedding settles and hair often looks slightly denser at the crown. Months six to twelve: this is when photographs start showing a real difference for responders. Beyond a year: the job becomes maintenance, and continuing to take it consistently is what protects the gains.
Side effects and safety monitoring
Most men take finasteride without problems, but a small proportion experience side effects, most commonly changes in sexual function, and less commonly low mood. The MHRA asks prescribers to make these risks clear before treatment starts, and the sensible rule is simple: if you notice changes that concern you, stop and speak to your prescriber promptly. Our full guide to finasteride side effects covers frequencies and what to do in detail.
Continued use matters
Finasteride manages an ongoing process rather than fixing it once. If you stop, DHT levels return to where they were and the loss usually resumes over the following months. We cover this in what happens when you stop finasteride. It is a prescription-only medicine, so a UK prescriber reviews whether it is appropriate for you before and during treatment.
