Platelet-rich plasma involves drawing your own blood, spinning it to concentrate the platelets, and injecting that plasma into the scalp or the dermis. The rationale is that growth factors held in platelets stimulate the tissue locally. Because the material comes from you, allergic reaction is not the concern; the concern is whether it works, and how well.
What the evidence supports
This is the part worth facing directly. Protocols vary widely between clinics: spin speed and duration, the concentration achieved, whether the platelets are activated, how many sessions are given and how far apart. Published studies are therefore hard to compare, often small, and mixed in their conclusions. Platelet-rich plasma is plausible and widely practised, but it does not carry the level of proof of an established medical treatment. Nobody can honestly quote you a regrowth percentage.
Who it suits
In hair loss it is better suited to early thinning or declining density, alongside medical treatment, than as a substitute for surgery in an area that is already bare. In dermatology it is offered for skin quality and for certain scars. Sessions are repeated, and any effect needs maintaining rather than being a single event.
Questions worth asking
Ask which protocol is used and why that one; how the result will be measured, ideally with standardised photographs taken in the same light and position; how many sessions are planned and what the total cost is. Ask as well what is proposed if nothing changes. A clear answer to that last question tells you a great deal about how the clinic operates.