Many of my patients have tried it or want to learn more about it. One of them, an educated, successful and anxious man in his 40s, recently told me he tried mixing CBD oil in his tea, but it didn’t make him calmer. Then he rubbed the oil on his injured knee, and pronounced it a magic cure.

Which invites the critical question: Just how effective is CBD, and for what kinds of ills?

Cannabidiol has little direct effect on the cannabinoid receptors in the brain, so it is largely devoid of the euphoric effects of THC, the major intoxicant in marijuana. But if CBD really had no psychotropic effect at all, it would be hard to understand its popularity. In fact, because it alters the brain’s serotonin receptors and may interfere with the breakdown of anandamide — a cannabidoid that is produced naturally in the brain — it could well affect feeling and thinking.

But what does the evidence show?

In 2017, the National Academies of Sciences, Engineering and Medicine convened a panel of experts to review the health effects of cannabis and cannabinoids. They examined more than 10,000 studies, most of which examined marijuana, not CBD. They found evidence that some cannabinoids — not including CBD — are effective for pain, nausea from chemotherapy and muscle spasms in multiple sclerosis .

When it comes to CBD, the panel found only a few small randomized clinical trials, and concluded that there was insufficient evidence that CBD was effective in treating conditions like insomnia, addiction to cigarettes and Parkinson’s disease , and limited evidence in its ability to treat anxiety.

This year, the Food and Drug Administration approved Epidiolex, a CBD concentrate, for two rare and severe forms of epilepsy, on the basis of several clinical trials.