Dr. Lester Grinspoon, a Harvard psychiatry professor who became a leading proponent of legalizing marijuana after his research found it was less toxic or addictive than alcohol or tobacco, died on June 25 at his home in Newton, Mass. He was 92.

His son David confirmed the death.

Dr. Grinspoon was an unlikely crusader for marijuana. At first, he believed that it was a dangerous drug. When the astronomer Carl Sagan, a friend who was also teaching at Harvard, offered him a joint in the late 1960s, Dr. Grinspoon warned him against continuing to smoke it.

“He took another puff and said, ‘Here, Lester, have some,’” he told The Boston Globe in 2018. “‘You’ll love it and it’s harmless.’ I was absolutely astonished.”

Dr. Sagan’s response was, in effect, a challenge. Dr. Grinspoon plunged into a review of existing research, hoping to find studies that agreed with his view of marijuana’s medical risks. He found that 19th-century physicians prescribed marijuana for pain and to help people sleep, but he found nothing to back decades of hysteria that marijuana was addictive, the view embodied in the lurid 1936 film “Reefer Madness” (originally called “Tell Your Children”) and the federal government’s decision to make it illegal a year later.

He concluded that marijuana was a relatively safe intoxicant that should be regulated like alcohol. The real danger, he said, was criminalizing its users.

After previewing his findings in an article in Scientific American in 1969, Dr. Grinspoon wrote “Marihuana Reconsidered.” It was published in 1971.

“The greatest potential for social harm lies in the scarring of so many young people and the reactive, institutional damages that are direct products of present marihuana laws,” Dr. Grinspoon wrote. “If we are to avoid having this harm reach the proportions of a real national disaster within the next decade, we must move to make the social use of marihuana legal.”

Reviewing the book in The New York Times, James L. Goddard, a former commissioner of the Food and Drug Administration, praised Dr. Grinspoon’s research.

“I can only express my admiration for the manner in which Grinspoon has extracted, analyzed and synthesized the most relevant literature to present the reader with a coherent, logical case,” Mr. Goddard wrote.

The review — its headline read, “The best dope on pot so far” — caught the attention of President Richard M. Nixon, who had begun to push a hard line on drugs. President Nixon circled Dr. Grinspoon’s name on a clipping of the review and wrote, “This clown is far on the left.”

The book was published a year after the founding of the National Organization for the Reform of Marijuana Laws, or NORML. Dr. Grinspoon, who would serve on the organization’s board of directors and its advisory board, became one of its scientific mentors, his book an intellectual road map to legalization.

“In the early days, he gave us incredible credibility,” Allen St. Pierre, a former executive director of NORML, said in an interview. “He showed there was a history to marijuana, that it hadn’t just been discovered by hippies in the 1960s. And by the time I came on the scene in 1990, Lester had achieved a high status in the marijuana reform movement; he was the person people respected the most.”

Dr. Grinspoon was a “scholarly, kind of nerdy guy,” his son David said in an interview. “That was part of his power when he got involved in the issue. He was a very professorial person, not a hippie.”

Dr. Grinspoon had not tried marijuana while acquiring his expertise in it. After his book was published, he defensively told some interviewers who were surprised by his restraint that he had also written a book on schizophrenia without having experienced it.

But he did relent. He and his wife, Betsy, tried marijuana twice in 1972, but were chagrined that they were unable to get high. On their third attempt, however, they listened to the Beatles’ album “Sgt. Pepper’s Lonely Hearts Club Band” — which Dr. Grinspoon had largely ignored in the past when his sons played rock music in the house — and achieved a thrilling high.

“It was for me a rhythmic explosion, a fascinating new musical experience!” he wrote in an essay on his website. “It was the opening of new musical vistas.”

Dr. Grinspoon told his story to John Lennon the night before testifying for him at a United States Immigration and Naturalization Service deportation hearing in 1972; pivotal to the government’s effort to deport Lennon was his earlier conviction in England for possessing cannabis resin, or hashish.

When Leon Wildes, Lennon’s lawyer, questioned Dr. Grinspoon, Mr. Wildes sought to prove that hashish was not a narcotic and that possessing it should not be cause for deportation. During his questioning, Mr. Wildes asked, “Dr. Grinspoon, is cannabis resin marijuana?”

“No, cannabis resin is not marijuana,” Dr. Grinspoon replied.

“Is cannabis resin a narcotic drug?” Mr. Wildes asked.

“No, cannabis resin is not a narcotic drug,” Dr. Grinspoon responded.

Lennon won the case when a federal appeals court ruled in 1975 that his British conviction was not enough to deport him.

Lester Grinspoon was born on June 24, 1928, in Newton. His father, Simon, a Russian immigrant, was a lawyer; his mother, Sally (Rose) Grinspoon, went to work as Isaac Asimov’s secretary at the Boston University School of Medicine after her husband’s death in 1949.

Concerned that his parents could not afford to send him to college, Lester dropped out of high school and joined the merchant marine. Then, although he did not have a high school diploma, he graduated from Tufts University, where he studied chemistry and biochemistry.

After graduating from Harvard Medical School in 1955, he served in the United States Public Health Service before starting his residency at the Massachusetts Mental Health Center in Boston. He then joined the center’s staff as a psychiatrist and stayed for 40 years. He also spent 42 years on the faculty of Harvard Medical School before retiring in 2000.

Dr. Grinspoon came to believe that Harvard’s discomfort with his marijuana activism led to the university’s refusal to promote him from associate professor of psychiatry to full professor. Although Harvard’s actions hurt him for a while, David Grinspoon said, his father “got to a point where he didn’t care about that.”

After finishing “Marihuana Reconsidered,” Dr. Grinspoon found a compelling personal reason to believe in pot’s medicinal value. His son Danny had been treated for acute lymphocytic leukemia for four years and in 1971 had begun taking a drug that caused severe nausea and vomiting.

Dr. Grinspoon’s wife, Betsy, suggested that Danny try marijuana. Dr. Grinspoon rejected the idea partly because it was illegal. But Ms. Grinspoon ignored him. On the day of Danny’s next treatment, she drove him to his high school, where she asked one of his friends in the schoolyard to get them some marijuana.

After smoking it in the hospital parking lot, Danny was suddenly free of the anxiety he had previously felt before taking the drug and experienced none of the prior side effects. His doctor encouraged him to smoke in his office before being treated with the drug at each subsequent chemotherapy session.

“From then on, he used marijuana before each treatment, and we were all much more comfortable during the remaining year of his life,” Dr. Grinspoon wrote in “Marihuana: The Forbidden Medicine,” the follow-up to “Marihuana Reconsidered,” written with James B. Bakalar and published in 1993.

The sequel “helped propel the serious legislative and legal issues surrounding the medicinal use of marijuana in California in the mid-1990s,” said Mr. St. Pierre, the former NORML executive.

Today, adult possession of marijuana is legal in 11 states and the District of Columbia, and 33 states and the District of Columbia let patients with a doctor’s supervision have access to marijuana or its plant-derived products at retail dispensaries.

Dr. Grinspoon married Evelyn Popky, known as Betsy, in 1954. In addition to her and their son David, he is survived by two other sons, Peter, a physician, and Joshua; five grandchildren; and two brothers, Harold and Kenneth.

After getting that first high while listening to the Beatles, Dr. Grinspoon continued to smoke marijuana with his wife and his sons (but only after they became adults). He also earned the distinction of having Barney’s Farm, a marijuana seed developer in the Netherlands, create a strain of cannabis called Dr. Grinspoon.

On its website, Barney’s Farm describes Dr. Grinspoon as offering “an old school sativa of the highest order, which gives a strong, long-lasting, energetic and cerebral high.”