The honest answer is: it depends on what we are treating and how your tissue responds. But there are useful patterns from the research and from experience that can help you plan.
For early to moderate knee arthritis, many studies use a series of two to three injections spaced about one to two weeks apart, and some patients do well with a single injection. In my practice, I often start with one well-prepared, well-placed injection and reassess at six to eight weeks. If you get a solid response, we may simply repeat it when the benefit starts to fade — often many months later — rather than committing to a series up front.
For tendon problems like patellar tendinitis or tennis elbow, the pattern is similar: one injection, then reassess. Tendons heal slowly, so we judge the result over weeks to a few months, not days. A second injection is reasonable if the first gave partial but incomplete relief. If two well-done injections have not moved the needle, it is time to rethink the diagnosis or the plan rather than keep repeating the same treatment.
Timing matters as much as number. PRP works by triggering a healing response, and that response needs the right environment: relative rest from the aggravating activity for a short window, then a gradual, guided return to loading — often with physical therapy. Injecting without fixing the load problem is why some people say “PRP didn't work.”
How do we know it worked? Not by the first week — it is normal to be sore for a few days after the injection. We look at six to twelve weeks: pain with daily activity, function, and whether you are back to the things you were missing. A good response usually lasts six months to a year or more for arthritis, and can be longer-lasting for tendons.
Every PRP treatment I offer is prepared from your own blood, in the office, on the same day. I do not use marketed “stem cell” products, placental tissue, Wharton's jelly, or exosomes — none of which are FDA-approved for these uses. Your own cells, only.
If you are weighing PRP, the most useful thing I can offer is a straight answer about whether your specific problem is likely to respond, and a clear plan for how many injections, how far apart, and how we will measure success before we start.
This article is general education, not a diagnosis or a treatment plan. If you have sudden swelling, locking, inability to bear weight, fever, or a hot red joint, seek urgent care promptly.