FORT MYERS, Fla. — With opening day around the corner, Cliff Lee didn't have time for another abdominal strain.

The former Cy Young Award winner had a similar injury twice before in spring training, and on both occasions it kept him out at least six weeks. This time, Lee was anxious to get back much sooner to help his new team, the Seattle Mariners.

So the pitcher chose an unusual treatment in which his blood was drawn, then a solution created from it injected back into his body. The technique, known as platelet-rich plasma injection therapy, has become trendy among top athletes — even though there's doubt in the medical community about whether it works.

"It's helped a lot of athletes speed up their healing process," said Lee, who had the treatment on March 19. "I'm hoping it does the same for me."

A recent study in the Netherlands found the treatment was no better than a placebo, the kind of conclusion reached about more common alternative therapies like ginkgo biloba (for memory) and glucosamine (arthritis) by the National Center for Complementary and Alternative Medicine.

Other athletes known to have undergone PRP therapy, also known as "blood spinning," include Pittsburgh Steelers receiver Hines Ward, golfer Scott Verplank, Denver Nuggets power forward Kenyon Martin and figure skater Patrick Chan.

The Associated Press spoke to seven pro athletes who have tried PRP therapy. While not a scientific sampling, none of them could definitively say that it improved their condition.

"It has not produced the results I was hoping for," said Verplank, a five-time winner on the PGA Tour who used it on his injured left elbow. "I wouldn't say it was a failure, but after six weeks, I didn't feel like I had a new elbow."

The procedure involves withdrawing about an ounce of blood and spinning it down for about 20 minutes in a centrifuge. That typically yields about a teaspoon of PRP, which is then injected directly into wounded tissue. The idea is the platelets contain proteins known as growth factors that are thought to promote cell growth and healing.

"It actually feels like a cramp," said Portland Trail Blazers All-Star Brandon Roy, who had the treatment for a hamstring injury. "They shoot it in there and my leg is sort of cramping."

PRP has been used in surgery and other fields for at least two decades to help with bone and tissue healing. More recently, sports medicine specialists have used PRP injections in outpatient settings as an alternative to surgery.

The therapy has gained notoriety in the past few months partly because of the case of Dr. Anthony Galea, a Canadian sports medicine doctor at the center of a drug investigation in both his country and the United States. Galea told The Associated Press last month that he has been "spinning blood for seven, eight years."

Its use by athletes also has stoked controversy because of allegations that some may have combined PRP with bioengineered human growth hormone, a banned substance in sports. Some agencies also consider PRP a potential performance-enhancer, although doctors who use it insist it merely helps the healing process.

The World Anti-Doping Agency, whose policies govern Olympic sports, has restricted PRP use because injections into muscle can have potential performance-enhancing effects. Injections into tendons and other tissues are permitted by WADA, but athletes must document that a treatment is for medical reasons.

Dr. Kenneth Mautner, a sports medicine specialist at Emory University, likens PRP treatment to fertilizing the lawn. "It's almost like you're jump-starting the healing process," he said.

Mautner insisted "there's absolutely nothing performance-enhancing with it." However, he cautioned that "it would be very easy in the wrong person's hands to add substances like growth hormone to the mixture."

Cases in which PRP is typically used include tendinitis in the elbow, knee and Achilles' tendon, but that list is growing. Braves reliever Takashi Saito underwent PRP treatment on his right elbow, avoiding surgery and helping the Los Angeles Dodgers to the playoffs in 2008.

Saito said through an interpreter that he wasn't sure if it worked because doctors later determined the injury was in a tendon higher in his arm and not in the elbow area.

"I never had a vision it would work for all types of problems," said Dr. Allen Mishra, an orthopedic surgeon at Stanford Medical Center and one of the first researchers on PRP.

PRP treatment is not approved by the Food and Drug Administration, and it's not cheap — each injection costs between $500 and $1,500. More than one injection is often needed and insurance usually doesn't cover the treatment because it's considered experimental.

Rigorous scientific evidence on its effectiveness is sparse. Several small studies have had mixed results, and even doctors who endorse PRP treatment say more extensive studies are needed to prove its usefulness.

"It's a promising treatment that has shown a lot of clinical success and is probably going to be used more and more in the future," Mautner said. "But I definitely think we need to make sure the science backs up all the excitement."

The Dutch study, published in the Journal of the American Medical Association, found that PRP patients with Achilles' tendon injuries fared no better than those who received a placebo injection of salt water. Both groups improved after six months.

Dr. Victor Ibrahim, a sports medicine specialist at the National Rehabilitation Hospital in Washington, D.C., who has used the procedure on professional and amateur athletes, claimed the study was flawed. He pointed to a small sample size — only 54 participants — and said injecting salt water could also cause inflammation that might help healing.

Ibrahim said PRP represents "a paradigm shift" in treating pain. Unlike rest, ice and other conventional methods to reduce swelling, PRP injections promote inflammation by revving up the body's natural response to healing. This in turn may help regenerate damaged tendons.

The New York Giants, D.C. United of Major League Soccer, and the San Diego Padres are among other pro sports teams that have had players give it a try.

New York Mets star Jose Reyes tried PRP to help a torn hamstring, though it didn't work.

"I had to get the surgery anyway, so it didn't do anything," Reyes said. "What can I say? I felt better for a few days, but as soon as I started running, it came back."

Dr. Scott Rodeo, an orthopedic surgeon at New York City's Hospital for Special Surgery, recently finished a randomized study that showed little difference between patients with rotator cuff injuries who got PRP versus those who did not.

Rodeo agreed the treatment has potential, but also said, "There's a lot we don't know about it," including what is the appropriate dose and timing of treatment to achieve the best results.

As for Lee, the Mariners shut him down last month in hopes that simple rest would get him healthy. Still, he won't be the Opening Day starter on Monday at Oakland.

"I don't think the injection did anything bad," Lee said. "I'm no scientist, but it's injecting my own blood back into me, so I don't see how it could be a bad thing."

Twins closer Joe Nathan did some exploring to see whether PRP could help him avoid surgery for a torn ulnar collateral ligament in his pitching elbow. But he decided to go the more proven method of Tommy John surgery and will miss the entire season.

"That's the new hot thing out right now, sure," Nathan said, "but it's still yet to be seen. How's it going to help a complete tear except maybe delay the inevitable? I could have had a temporary quick fix and then three months later be back to square one."

AP Medical Writer Lindsey Tanner in Chicago, AP Golf Writer Doug Ferguson in Orlando, Fla., and AP Sports Writers Gregg Bell in Seattle, Joseph White in Washington, D.C., Anne M. Peterson in Portland, Ore., and Tom Canavan in East Rutherford, N.J., contributed to this story.