Finasteride works by blocking type 2 5-alpha reductase, the enzyme that converts testosterone into dihydrotestosterone. Less DHT at the follicle means less miniaturisation, and over months that shows up as more hairs staying in the growth phase.
The tablet does this very well — and it does it everywhere in the body, not only in the scalp. That is both why it works and why some men refuse it. A topical spray is an attempt to keep the drug where the problem is.
What the trial actually did
Four hundred and fifty-eight men with androgenetic alopecia, forty-five centres in Europe, randomised double-blind against placebo with a double-dummy design so that neither patient nor investigator knew which treatment was active. Twenty-four weeks. Three hundred and twenty-three men completed; 446 were included in the safety analysis.
The primary endpoint was target area hair count — the number of hairs inside a small, fixed circle of scalp, counted from standardised macro photographs. It is the field's standard measuring stick, and it is also a surrogate: more hairs in a circle is not the same thing as a better-looking head of hair.
The result, without the adjectives
At 24 weeks the treated group had gained a mean of 20.2 hairs in the target area. The placebo group gained 6.7. The difference was statistically significant (p < 0.001), and the authors describe the result as numerically similar to what the oral tablet achieves in comparable studies.
Note the placebo number. Any scalp study that involves photographing, measuring and touching the head produces some growth in the control arm. A trial that reports only the treated figure is telling you half the story.