Do PRP injections work?

Short answer

Not for tendon or knee pain. In trials against a saltwater fake shot, pain and function did not improve more. The one confirmed gain was getting back to sport about 1 week sooner after a muscle injury.

Do PRP injections work?
'Healing factors from your own blood → tissue repair' — does this formula hold?
PRP means drawing your blood, concentrating the platelets, and injecting them back into the painful spot. The pitch is that healing factors in platelets repair tendons and joints. We checked trials that pitted it against a fake shot of saltwater.
Start with tendon pain
Pooling 13 trials, no difference from a fake shot
An analysis pooled 13 trials comparing PRP with a fake shot for tendon pain. At 4-6 weeks, 12 weeks, and 24 weeks or more, pain and function did not improve more than with the fake shot. Splitting by tendon, number of shots, or other treatments made no difference.
Achilles tendon
240 people, scores of 54.4 vs 53.4 at 6 months
240 people with Achilles pain lasting over 3 months got either PRP or a fake needle that never touched the tendon. At 6 months, symptom scores (100 = no symptoms) were 54.4 and 53.4, no difference. An analysis pooling 4 trials with 337 people also found no difference in scores or tendon thickness.
Tennis elbow
A 0.2-point pain gap out of 10 after 1 year
119 people with outer elbow pain got one shot of PRP, their own blood, or saltwater. After 1 year, the pain gap between PRP and saltwater was 0.2 points out of 10. The arm function gap was 0 points and the grip gap was 1.4 kg, neither a meaningful difference.
Knee arthritis
Saltwater also cut pain by 1.8 points
288 people with knee arthritis got 3 shots of PRP or saltwater, one week apart. After 12 months, pain out of 10 dropped 2.1 points with PRP and 1.8 points with saltwater, no difference. Cartilage volume fell 1.4% and 1.2%, also no difference.
What about a torn muscle?
Back about 1 week sooner, same reinjury rate
An analysis pooling 9 trials with 474 people with muscle injuries found PRP got them back to sport 7.5 days sooner. In the 4 trials (217 people) where neither patients nor researchers knew who got what, it was 8.8 days. Reinjury did not drop (15% vs 16% across 6 hamstring trials).
Bottom line: no effect for tendons or knees
The confirmed gain is about 1 week faster return after muscle injury
For the Achilles, elbow, and knee, PRP did not improve pain or function more than saltwater or a fake needle. The pain relief people felt after the shot showed up with saltwater too. After muscle injuries, return was about 1 week faster, but reinjury did not drop.