When your doctor gives you health advice, and your insurer pays for the recommended treatment, you probably presume it’s based on solid evidence. But a great deal of clinical practice that’s covered by private insurers and public programs isn’t.

The British Medical Journal sifted through the evidence for thousands of medical treatments to assess which are beneficial and which aren’t. According to the analysis, there is evidence of some benefit for just over 40 percent of them. Only 3 percent are ineffective or harmful; a further 6 percent are unlikely to be helpful. But a whopping 50 percent are of unknown effectiveness. We haven’t done the studies.

Sometimes uncertain and experimental treatments are warranted; patients may even welcome them. When there is no known cure for a fatal or severely debilitating health condition, trying something uncertain — as evidence is gathered — is a reasonable approach, provided the patient is informed and consents.

“We have lots of effective treatments, many of which were originally experimental,” said Dr. Jason H. Wasfy, an assistant professor of medicine at Harvard Medical School and a cardiologist at Massachusetts General Hospital. “But not every experimental treatment ends up effective, and many aren’t better than existing alternatives. It’s important to collect and analyze the evidence so we can stop doing things that don’t work to minimize patient harm.”