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Tiger Woods, Chris Canty (a defensive tackle for the New York Giants) and the Phillies' pitcher Cliff Lee have in the past year or so greatly boosted the popularity of a controversial therapy by employing it to combat a sore knee (Woods), hamstring (Canty) and abdomen (Lee). The treatment, platelet-rich plasma therapy, or P.R.P., involves centrifuging a person's own blood until it contains a concentrated mix of plasma cells and growth factors and then injecting the resulting substance directly into the injured tissue. In theory, the distilled growth factors (a protein or substance that helps stimulate growth) should speed healing and improve the tissue's health, which has happened in the lab. When scientists surgically created lesions in animal tendons and other tissues, P.R.P. therapy nudged the injured tissues to rapidly create new collagen and blood vessels. Meanwhile, testimonials from professional athletes suggested that the shots work in humans as well. Today, recreational athletes reportedly clamor for P.R.P. to treat everything from tennis elbow to back pain, even though the procedure rarely is covered by insurance and can cost $1,000 a shot. "It has buzz," said Dennis Cardone, a clinical associate professor at New York University's Langone Medical Center who has written about the use of P.R.P. in sports medicine.

But now, rather belatedly, science is showing up to spoil the fun. Several new studies have examined whether P.R.P. is effective outside the lab, and as Leon Creaney, a sports-medicine consultant in London and the author of one of the papers, said, "the evidence has not been favorable" for P.R.P.

Phys Ed

Perhaps the most telling of the new studies, by Dr. Creaney and his colleagues, has been accepted by The British Journal of Sports Medicine and will published online soon. In it, scientists treated people suffering from refractory tennis elbow with either P.R.P. or injections of whole blood. Whole blood contains far fewer growth factors than P.R.P. Presumably, then, injections of blood would not accelerate and amplify healing in the same way as P.R.P. But the whole blood turned out to be as effective as P.R.P. at treating tennis elbow after three months, and more so at six months. Both treatments reduced pain in most volunteers (whose tennis elbows had not responded to physical therapy). But in the end, the lower concentration of platelets and growth factors in the whole blood was better. " 'Less,' " the authors write, "may in fact be 'more.' "

This finding is in line with that of another study reported this month in The British Journal of Sports Medicine. The sequel to a much-discussed experiment from last year, it re-examined patients with Achilles tendinopathy (an overuse injury of the Achilles tendon) who had been randomly assigned to receive injections of either P.R.P. or a placebo of saline solution. In the original study, the patients were assessed after six months and the researchers found no statistical difference between the two groups' recoveries. Now an additional six months later (meaning a year after treatment), the results were the same. Salt water worked as well as high-tech P.R.P., prompting the authors to conclude that there is "no evidence for the use of platelet-rich plasma injection in chronic Achilles tendinopathy."

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Why, then, has P.R.P. received so much praise from early adopters, and what are we to make of results like those from yet another report in the same issue of The British Journal of Sports Medicine, in which P.R.P. improved the healing of man-made lesions in lab rats' tendons?

"This seems contradictory," admitted Robert-Jan de Vos, a researcher at the Erasmus University medical center in the Netherlands and lead author of the Achilles tendinopathy study. But, he said, there are fundamental differences between overuse injuries, such as tendinopathies, and acute wounds, like those created in animal experiments. In acute injuries, the body initiates a robust healing response, which, Dr. de Vos said, P.R.P. may intensify. But in overuse injuries, the healing process is often blunted, and P.R.P. seems unable to augment it much.

That's only a theory, however. In general, the mechanisms by which P.R.P. succeeds or fails remain unexplained. "We believe more work on the basic science needs to be undertaken," a consensus statement about P.R.P. issued late last year by the International Olympic Committee said, adding that people should "proceed with caution in the use of P.R.P. in athletic sporting injuries."

So what is someone with a sore ankle, knee, elbow, shoulder, abdominal muscle or other sports injury to do? "First, exhaust the standard treatments," said Dr. Cardone. Begin with physical therapy, he said, which has a proven track record against overuse injuries and is much less expensive and invasive than P.R.P.

"The injections are very painful," Dr. de Vos pointed out.

Only if other treatments are unsuccessful, Dr. Cardone continued, should you consider P.R.P. or other shots, like whole blood. (Some researchers suspect that you get benefits, if any, from the needle prick, not the contents of the syringe. The needle causes a small amount of bleeding in the injured tissue, which could potentially initiate a healing response from the body.) "It's worth trying P.R.P. before turning to surgery," Dr. Cardone said. Most of the people in the latest studies did recover after receiving P.R.P., he pointed out, although no better than with other, cheaper injections. So if you're undeterred by the expense, discomfort and questionable utility of P.R.P., the treatment may be worth the risk, when almost all else has failed. "Based on what we know at this point," Dr. Cardone concluded, "I'd say that P.R.P. is probably best reserved as a last resort before the final last resort of surgery."