David McKeever said he was smoking "good stuff" — potent marijuana supplied by a friend — when he experienced a vivid hallucination four years ago that his longtime companion, Delma Murphy, was "a mystic with blue hair, fiery red eyes, who was voluptuous and naked, save for her translucent cape."

He saw Murphy as a demon as they fought in their New London home, McKeever told clinicians at the Whiting Forensic Institute, and knew in an instant that he was going to kill her.

McKeever stabbed Murphy repeatedly with a hunting knife and, though he knew she was dead, he said she continued to talk to him over the next several days. He also heard directly from God, Angelina Jolie, a character from the cartoon "Family Guy" and Brad Pitt, McKeever reported.

McKeever didn't blame the marijuana as much as he blamed Murphy during the interviews at Whiting, but a social worker, psychiatrist and psychologist who evaluated him concluded McKeever committed the crime during a cannabis-induced psychosis that persisted for about two months after he was charged with murder.

Marijuana, an increasingly available and potent source of healing and pleasure for some, can induce psychosis in a segment of the population, according to medical experts.

"Many studies show your risk of psychosis or schizophrenia is somewhere between two to four times higher than the general population if you use marijuana," said Dr. Deepak Cyril D'Souza, a professor of psychiatry at Yale University, staff physician in the Veterans Administration Connecticut Healthcare System and member of the state's Medical Marijuana Program board of physicians.

D'Souza said exposure to marijuana at an early age and frequency of use increases the risk of developing schizophrenia. Those who have a family history of psychosis might be at an even greater risk, he said. Not everyone who smokes cannabis develops schizophrenia, and not everyone who has schizophrenia has smoked marijuana, he said.

The connection between marijuana and psychotic reactions, such as delusions, hallucinations, talking incoherently and agitation, is not a new story, according to D'Souza. He cited several studies, starting with French physician Jacques Joseph Moreau's 1845 work on hashish users presenting with symptoms of schizophrenia, to a massive modern study of 50,087 people serving mandatory time in the Swedish military.

D'Souza said he was familiar with recent news articles linking marijuana, mental illness and violence, but said the overwhelming majority of mentally ill people he's worked with in 25 years of practicing medicine have been nonviolent.

"That said, sometimes when people experience a worsening of psychosis, they could become violent, and one factor that could contribute to a worsening psychosis is substances, and one substance is cannabis," D'Souza said.

An opinion piece titled "What Advocates of Legalizing Pot Don't Want You to Know," which appeared recently in the New York Times, suggests that marijuana use leads to an increase in violent crime and that the first four states to legalize marijuana — Alaska, Colorado, Oregon and Washington — have seen sharp increases in murders and aggravated assaults. The author, former New York Times reporter Alex Berenson, has written about the issue in a book, "Tell Your Children: The Truth About Marijuana, Mental Illness, and Violence," according to the article.

In McKeever's case, the Whiting forensic team concluded he may have been predisposed to marijuana-induced psychosis due to a personality disorder and a congenital condition known as Usher Syndrome, which is associated with higher rates of psychosis than what typically occurs in the general population. McKeever had started drinking alcohol at age 12 and smoking marijuana at 13, according to a forensic report filed in New London Superior Court. He said he'd used methamphetamine, LSD, marijuana laced with heroin, cocaine and K2, a type of synthetic marijuana that the American Psychiatric Associations say can cause long-lasting psychotic behavior. The report indicates McKeever had one previous hospitalization for marijuana-induced psychosis.

The chemical content of the marijuana that McKeever smoked before and after he killed Murphy is unknown. The friend who had brought the marijuana to the couple's house described it as "good stuff" that had made her very high, according to the court document.

"With consistent abstinence from illicit substances since he was arrested in November 2015, Mr. McKeever's mental state has improved significantly," the report says.

McKeever, who is 51, was found not guilty by reason of insanity and committed to the care of the state's Psychiatric Security Review Board for 60 years.

With the growing number of medical marijuana patients in Connecticut — 30,761 as of Feb. 3, according to the state Department of Consumer Protection, and proposed legislation legalizing marijuana for recreational use, D'Souza said the state owes it to citizens to set up a way to monitor new cases of psychosis and new reports of cannabis use disorder — or the problematic use of marijuana — as well as the number of hospitalizations related to cannabis use and the cost.

He also cited a need to track motor vehicle accidents resulting from driving under the influence of marijuana.

"My focus is really on the evidence," D'Souza said. "I don't have any political view. If marijuana works for certain conditions, I'm all for that. I think we need evidence of it."

Certain forms of cannabis are more likely to be associated with psychosis, he said, such as strong strains known as "skunk" that have a high content of THC, which is the psychoactive, or intoxicating ingredient, in marijuana, and a lower content of CBD, which does not produce a high.

"CBD may offset some of the negative effects," he said.

k.florin@theday.com