A few years ago, the National Academy of Medicine convened a panel of sixteen leading medical experts to analyze the scientific literature on cannabis. The report they prepared, which came out in January of 2017, runs to four hundred and sixty-eight pages. It contains no bombshells or surprises, which perhaps explains why it went largely unnoticed. It simply stated, over and over again, that a drug North Americans have become enthusiastic about remains a mystery.

For example, smoking pot is widely supposed to diminish the nausea associated with chemotherapy. But, the panel pointed out, “there are no good-quality randomized trials investigating this option.” We have evidence for marijuana as a treatment for pain, but “very little is known about the efficacy, dose, routes of administration, or side effects of commonly used and commercially available cannabis products in the United States.” The caveats continue. Is it good for epilepsy? “Insufficient evidence.” Tourette’s syndrome? Limited evidence. A.L.S., Huntington’s, and Parkinson’s? Insufficient evidence. Irritable-bowel syndrome? Insufficient evidence. Dementia and glaucoma? Probably not. Anxiety? Maybe. Depression? Probably not.

Then come Chapters 5 through 13, the heart of the report, which concern marijuana’s potential risks. The haze of uncertainty continues. Does the use of cannabis increase the likelihood of fatal car accidents? Yes. By how much? Unclear. Does it affect motivation and cognition? Hard to say, but probably. Does it affect employment prospects? Probably. Will it impair academic achievement? Limited evidence. This goes on for pages.

We need proper studies, the panel concluded, on the health effects of cannabis on children and teen-agers and pregnant women and breast-feeding mothers and “older populations” and “heavy cannabis users”; in other words, on everyone except the college student who smokes a joint once a month. The panel also called for investigation into “the pharmacokinetic and pharmacodynamic properties of cannabis, modes of delivery, different concentrations, in various populations, including the dose-response relationships of cannabis and THC or other cannabinoids.”

Figuring out the “dose-response relationship” of a new compound is something a pharmaceutical company does from the start of trials in human subjects, as it prepares a new drug application for the F.D.A. Too little of a powerful drug means that it won’t work. Too much means that it might do more harm than good. The amount of active ingredient in a pill and the metabolic path that the ingredient takes after it enters your body—these are things that drugmakers will have painstakingly mapped out before the product comes on the market, with a tractor-trailer full of supporting documentation.

With marijuana, apparently, we’re still waiting for this information. It’s hard to study a substance that until very recently has been almost universally illegal. And the few studies we do have were done mostly in the nineteen-eighties and nineties, when cannabis was not nearly as potent as it is now. Because of recent developments in plant breeding and growing techniques, the typical concentration of THC, the psychoactive ingredient in marijuana, has gone from the low single digits to more than twenty per cent—from a swig of near-beer to a tequila shot.

Are users smoking less, to compensate for the drug’s new potency? Or simply getting more stoned, more quickly? Is high-potency cannabis more of a problem for younger users or for older ones? For some drugs, the dose-response curve is linear: twice the dose creates twice the effect. For other drugs, it’s nonlinear: twice the dose can increase the effect tenfold, or hardly at all. Which is true for cannabis? It also matters, of course, how cannabis is consumed. It can be smoked, vaped, eaten, or applied to the skin. How are absorption patterns affected?

Last May, not long before Canada legalized the recreational use of marijuana, Beau Kilmer, a drug-policy expert with the RAND Corporation, testified before the Canadian Parliament. He warned that the fastest-growing segment of the legal market in Washington State was extracts for inhalation, and that the mean THC concentration for those products was more than sixty-five per cent. “We know little about the health consequences—risks and benefits—of many of the cannabis products likely to be sold in nonmedical markets,” he said. Nor did we know how higher-potency products would affect THC consumption.

When it comes to cannabis, the best-case scenario is that we will muddle through, learning more about its true effects as we go along and adapting as needed—the way, say, the once extraordinarily lethal innovation of the automobile has been gradually tamed in the course of its history. For those curious about the worst-case scenario, Alex Berenson has written a short manifesto, “Tell Your Children: The Truth About Marijuana, Mental Illness, and Violence.”

Berenson begins his book with an account of a conversation he had with his wife, a psychiatrist who specializes in treating mentally ill criminals. They were discussing one of the many grim cases that cross her desk—“the usual horror story, somebody who’d cut up his grandmother or set fire to his apartment.” Then his wife said something like “Of course, he was high, been smoking pot his whole life.”

Of course? I said. Yeah, they all smoke. Well . . . other things too, right? Sometimes. But they all smoke.

Berenson used to be an investigative reporter for the Times, where he covered, among other things, health care and the pharmaceutical industry. Then he left the paper to write a popular series of thrillers. At the time of his conversation with his wife, he had the typical layman’s view of cannabis, which is that it is largely benign. His wife’s remark alarmed him, and he set out to educate himself. Berenson is constrained by the same problem the National Academy of Medicine faced—that, when it comes to marijuana, we really don’t know very much. But he has a reporter’s tenacity, a novelist’s imagination, and an outsider’s knack for asking intemperate questions. The result is disturbing.

The first of Berenson’s questions concerns what has long been the most worrisome point about cannabis: its association with mental illness. Many people with serious psychiatric illness smoke lots of pot. The marijuana lobby typically responds to this fact by saying that pot-smoking is a response to mental illness, not the cause of it—that people with psychiatric issues use marijuana to self-medicate. That is only partly true. In some cases, heavy cannabis use does seem to cause mental illness. As the National Academy panel declared, in one of its few unequivocal conclusions, “Cannabis use is likely to increase the risk of developing schizophrenia and other psychoses; the higher the use, the greater the risk.”

Berenson thinks that we are far too sanguine about this link. He wonders how large the risk is, and what might be behind it. In one of the most fascinating sections of “Tell Your Children,” he sits down with Erik Messamore, a psychiatrist who specializes in neuropharmacology and in the treatment of schizophrenia. Messamore reports that, following the recent rise in marijuana use in the U.S. (it has almost doubled in the past two decades, not necessarily as the result of legal reforms), he has begun to see a new kind of patient: older, and not from the marginalized communities that his patients usually come from. These are otherwise stable middle-class professionals. Berenson writes, “A surprising number of them seemed to have used only cannabis and no other drugs before their breaks. The disease they’d developed looked like schizophrenia, but it had developed later—and their prognosis seemed to be worse. Their delusions and paranoia hardly responded to antipsychotics.”