How long does PRP last?

HOW LONG

The question people mean is not how long the platelets survive. It is how long the change they produced holds — and those are different clocks with different answers.

The platelets are gone in days

Injected platelets degranulate within hours and are cleared within days. If PRP worked by the platelets staying put, the effect would end that week. It does not, which tells you the mechanism is not the platelets themselves but what they signal — a shift in the local environment toward repair, changes in synovial behavior, and recruitment of the cells that actually do the work.

That is why the effect takes weeks to arrive and why it often gets worse before it gets better. You are not waiting for a drug to take effect. You are waiting for tissue to respond.

What the comparative trials show

The most informative comparison is against corticosteroid, because the shapes of the two curves differ rather than just the heights. Steroid works faster and fades; PRP is slower to arrive and tends to still be doing something at six and twelve months. Meta-analyses comparing PRP with hyaluronic acid in knee osteoarthritis point the same way — the separation between the two widens at the later time points rather than the early ones.

So a reasonable expectation for an appropriately selected knee is improvement measured in months rather than weeks, commonly still present at a year, and not permanent. One of these works faster and the other tends to still be working.

Why “how long” is the wrong frame for a tendon

In a joint, PRP modifies an environment that continues to be loaded, so the effect decays as the joint keeps doing what it was doing. In a tendon the goal is different: you are trying to convert a failed healing response into a completed one. When that works, the question of duration partly dissolves — a tendon that has remodeled is not on a countdown.

When it does not work, repeating the injection without changing the load that caused it is the common error. The tendon is not inflamed, and the loading pattern is doing more than the syringe.

What shortens it

• Continuing the load that caused it, unchanged. The most common reason a good result does not hold.
• Advanced structural disease. A joint with very little cartilage left has less to work with, and honesty about that is part of the consent. A replacement is a very good operation, and timing it is your decision.
• Metabolic terrain. Insulin resistance and poor glycemic control measurably impair tendon and connective-tissue healing. This is the variable most often left out and the one most open to change. It is why we look at your bloodwork.

And one thing that does not shorten it: having had the injection before. Repeat treatment is a clinical decision based on what happened, not a diminishing-returns rule.