What the evidence actually shows
This section exists because PRP is marketed far ahead of what is known about it.
What most studies find. In androgenetic alopecia, which is ordinary male and female pattern hair loss, the majority of published trials report a modest increase in hair density and hair thickness after a course of PRP compared with placebo or baseline. Reviews generally conclude there is a positive effect and that the quality of evidence is low to moderate.
Why the results conflict. There is no standard PRP. Studies differ in the volume of blood drawn, the centrifuge speed and number of spins, whether an anticoagulant is used, whether the platelets are activated with calcium chloride or thrombin, the final platelet concentration, the injection depth and pattern, the number of sessions and the interval between them. Two clinics can both offer PRP and deliver materially different preparations. This is the single biggest weakness in the literature and the main reason no one can tell you exactly what to expect.
Where it works best. Early to moderate pattern loss where follicles are miniaturising but present. Diffuse thinning in women. Alongside medication. And around a transplant.
Where it does not work. A bald scalp. Once follicles are gone, no injection restores them, and a clinic offering PRP to fill a bald crown is selling you something that cannot work.
Compared with medication. Finasteride and topical minoxidil have decades of trial data, clear licensing and larger effects. PRP sits below both. Any honest conversation about PRP starts with whether you are on medication, because that is where the larger benefit lies.
Other conditions. There is some evidence in alopecia areata, an autoimmune patchy hair loss, though that is primarily treated medically by dermatologists. Evidence in scarring alopecias is weak and those conditions need diagnosis and medical control first.
What a good clinic says. That it is an adjunct with modest, temporary effect, that a course and maintenance are needed, that results vary, and that medication comes first. What a poor clinic says is that it regrows hair, that it is a natural alternative to drugs, or that one session will do.
Ask the clinic what protocol they use: blood volume, spins, whether platelets are activated, and the platelet concentration. A clinic that can answer is doing this properly.