Free speech has a special virtue; it improves the chances that errors will be corrected. In a blog from early October, I praised the New York Times for its enterprise journalism focusing on the dangers of marijuana.

I thought these were new ideas, signaling to doctors the risks of marijuana. I was wrong. The research they assembled repeats the tired arguments of those worrywarts who see mostly danger from pot.

Peter Grinspoon, a psychiatrist at Massachusetts General Hospital and Harvard Medical School, has written a book challenging the pessimists who see dangers from marijuana use. Like his father, Lester, whose famous 1971 book “Marihuana Reconsidered” was a foundational text of the drug reform movement, Peter argues that on balance the positives of marijuana are greater than the risks.

In his book, Seeing through the Smoke: A Cannabis Specialist Untangles the Truth about Marijuana, Dr. Grinspoon confronts those researchers, such as the ones the NY Times interviewed, who see grave risks and little benefit from the legalization of cannabis.

He starts the book with a family story demonstrating that the plant is medicine. It’s a tale that the researchers interviewed by the NY Times would find impossible to refute. Peter’s older brother developed a blood cancer requiring massive chemotherapy, but it couldn’t arrest the spread of leukemia. Peter asserts, convincingly, that cannabis kept his brother alive for months. It performed this task by combatting the side effects of chemotherapy.

“Without cannabis, Danny would be lying in his room with a towel over his head and a barf bucket next to his bed at the ready. With cannabis, he would be downstairs playing board games and wrestling with his younger twin brothers.” “Instead of barfing, he was eating.…the improvement in his quality of life was incalculable.”

Without a doubt, marijuana added months to the life of his brother. Surely, this effect is medicinal; it minimized the side effects of chemotherapy.

What angers Dr. Grinspoon is that throughout history the medical profession has recognized that tinctures of marijuana and the marijuana plant treat certain illnesses. Migraine headaches is just one example. When Congress took a conservative turn after the reelection of Franklin Delano Roosevelt in 1936, southern conservatives formed a voting bloc with Republicans. This coalition had a working majority that lasted decades. In 1937, Congress made marijuana illegal for the first time in medical history. The American Medical Association opposed the legislation, but the government agencies that had enforced alcohol prohibition and their scary stories convinced the lawmakers. Pot quickly became an illegal pleasure, and a new market monopoly was given to lawbreakers.

This change in attitude was stunning. Pharmacists and doctors for centuries had gotten positive results from medicine using cannabis. It is one of many benefits that humans have gotten from the hemp plant. Throughout human history, they have used the hemp plant for practical uses rope, shoes, and medicine. Archeologists have found artifacts that use hemp as long ago as ten thousand years.

Pessimistic medical researchers limit their research by focusing on memory and other work-related mental tasks. A comprehensive report in 2017 concluded “there is strong data for immediate impairment, but little to no data for lasting impairment.” The negative conclusions relied on selective data; had the researchers “looked at, creativity, humor, and insight,” Grinspoon suspects “there wouldn’t have been deficits, and people might have done significantly better than the norm.” In short, the pessimists are desperately hunting for reasons to conclude pot is bad.

Unsurprisingly, Peter Grinspoon offers a different conclusion: pot, like alcohol and food, works best if used in moderation. However, the pessimists are creating a real danger: ignorance. Too many doctors lack an understanding of the properties for good and bad of marijuana. They are unable to help their patients who use cannabis and can’t recommend this drug even when the benefits are clear.

Grinspoon’s book Seeing Through the Smoke seeks to help doctors and the public understand how they can get benefits from pot.  As a physician, he advises that pot sold in legal markets is tested and users are not exposed to “mold, heavy metals, and other contaminants.” It is a much safer product than the illegal substance.

Peter Grinspoon is trying to create a common language and values that allow the public to make informed choices and to create a common understanding that will lead to a new consensus.