But the study leaves the options for some patients unclear.

“We don’t have a lot of good things in our toolbox for spinal stenosis,” Dr. Baker said. “We’re really stuck with a problem” with an aging population.

Spinal injections are considered effective for other conditions, like herniated discs. But of the 2.2 million given annually to people on Medicare, about a quarter are for spinal stenosis, said Dr. Janna Friedly, a professor of rehabilitation medicine at the University of Washington and the study’s lead author. She said injections cost $500 to $2,000 each. The study helps answer questions raised by the Spine Society and the Cochrane Collaboration, a group of medical experts. Both issued reviews last year finding insufficient evidence to recommend injections for some types of stenosis.

“If the benefit really isn’t there and you do the procedure more and more, then all you’re doing is compounding the risk,” said Dr. Christopher Standaert, a co-author of the study and a professor at the University of Washington.

Dr. Scott Kreiner, a co-chairman of the Spine Society’s evidence-based guidelines committee, said the new research should “influence future guidelines.”

A rehabilitation medicine specialist in Phoenix who was not involved in the study, Dr. Kreiner says he will give fewer second injections and may refer some patients to surgery sooner. “This is probably a step toward eliminating or minimizing the use of epidural steroid injections for this problem,” he said.

Still, the research, funded by the federal Agency for Healthcare Research and Quality, leaves questions unanswered.

Because every patient received injections, and both groups reported similar improvement six weeks later, researchers cannot tell if patients would do as well without any injections at all. Also unclear is whether the anesthetic, lidocaine, did anything helpful when injected alone. Some experts said the benefits patients reported seemed larger than typical placebo effects.