Is there any benefit or side effect of PRP on scalp? I am losing my hair and have scalp itching.

PRP treatments are fast growing in popularity. While there is a good deal of anecdotal data on its positive effects, there is very little randomized controlled (i.e. high quality) data available. With respect to hair restoration, proponent have demonstrated impacts ranging from slowing/cessation of loss to thickening/strengthening of hair. Some will claim to have demonstrated regrowth.
Ultimately, there is very little drawback or risk to the procedure with the potential for significant reward. While the mechanism of action is not entirely clear, the effect will likely be rooted in the native concentration of growth factors in any given patient's plasma. Some will have better quality or more concentrated plasma than others. The results range from decreased hair loss to thickening of existing or emerging hairs to increased growth. Ultimately, the variability in response is likely linked to the inherent (i.e. genetic) quality of the plasma which will differ from one patient to the next.
PRP has (anecdotally) proven to be effective in the arena of hair restoration both as an adjunct to grafting as well as standalone treatment for those who may not (yet) be a candidate for grafting. Patients have demonstrated slowed loss, thickening of existing hair and regrowth. The typical duration of effect is 6-9 months with most patients being treated 2x yearly (at least initially).

In my experience, it is a nice option for young people who have yet to reach a stable hair pattern (i.e. not ready for FUE grafting/Neograft) and are looking for an adjunct to traditional medical therapy. That being said, it is important to limit your expectations as not everyone experiences regrowth at the same pace (or at all). However, anecdotally speaking, everyone seems to gain some benefit with regular treatments (annually to bi-annually after 3 treatment induction).
With regards to your specific question, it is unclear if PRP will address pruritis/itching. This might warrant dermatologic evaluation to rule out other causes.
As always, discuss your concerns with a board certified plastic surgeon (ABPS), with experience in hair restoration.Donovan Rosas MD
Board Certified by the American Board of Plastic Surgery
Member: American Society for Aesthetic Plastic
Surgery, American Society of Plastic Surgeons
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PRP has many wonderful benefits for hair loss.  The  growth factors present in patient derived platelet rich plasma stimulate hair to stay in the growing phase which improves density and thus the appearance of hair.  It also has anti inflammatory properties and can improve the blood supply to support hair follicle growth.  There are minimal side effects and risks.  Some patients may experience mild swelling and feel a heaviness post injection but this subsides quickly.  In your particular case I would recommend seeing a qualified physician to evaluate your hair and address all of your concerns.