When Braxton Clark was in high school, he used marijuana to control his emotions. At 17, he used it every day. 

When he was 18, he had a psychotic episode after using cannabis and was admitted to a hospital. He spent the next three years sober. Then one day he tried cannabis again. Before long, he was back in the hospital. 

“I had lost my faculties. I wasn’t making sense,” said Clark, now 24.

He has been sober a year and is thriving in college with the help of medication. Doctors have diagnosed him with a psychotic disorder, brought on by using cannabis. 

Braxton is among thousands of teenagers and young adults who have developed delusions and paranoia after using cannabis. Legalization efforts have made cannabis more readily available in much of the country. More frequent use of marijuana that is many times as potent as strains common three decades ago is leading to more psychotic episodes, according to doctors and recent research. 

“This isn’t the cannabis of 20, 30 years ago,” said Dr. Deepali Gershan, an addiction psychiatrist at Compass Health Center in Northbrook, Ill. Up to 20% of her caseload is patients for whom she suspects cannabis use triggered a psychotic episode.

Rates of diagnoses for cannabis-induced disorders were more than 50% higher at the end of November than in 2019, healthcare-analytics company Truveta said this week. The trend is contributing to the broader burden of caring for people who developed mental health and addiction problems during the pandemic.

Symptoms of serious mental disorders including schizophrenia often emerge in adolescence. Cannabis can’t be isolated as the culprit in any particular case, but large studies show a clear link between frequent and more potent cannabis use and higher rates of psychosis, particularly in young users, said Dr. Deepak D’Souza, professor of psychiatry at Yale University School of Medicine. 

Even one psychotic episode following cannabis use was associated with a 47% chance of a person developing schizophrenia or bipolar disorder, a 2017 study in the American Journal of Psychiatry showed. The risk was highest for people 16-to-25-years-old and higher than for substances including amphetamines, hallucinogens, opioids and alcohol. 

At Boston Children’s Hospital, doctors are treating more children developing psychotic disorders from cannabis use. Nearly a third of adolescents they see for checkups say they are using cannabis. About a third of children using cannabis report experiencing hallucinations or paranoia.

Doctors and other health workers from the hospital’s Adolescent Substance Use and Addiction Program hold weekly rounds to review cases. Recently, they discussed one young cannabis user who thought she was being followed. One young man had nearly crashed his car because he thought demons were chasing him. A teenager with cannabis-use disorder had threatened to kill his mother. 

“This is a lot of my life, figuring out what to do with these kids,” said Dr. Sharon Levy, the hospital’s head of addiction medicine. 

Until recently, marijuana referred to plant material. These days it can mean plant extract containing highly concentrated THC, the substance responsible for marijuana’s intoxicating effects, or lab-created derivatives that were rare a couple of years ago.

The average THC content of cannabis seized by the Drug Enforcement Administration was 15% in 2021, up from 4% in 1995. Many products advertise THC concentrations of up to 90%.

“This is attacking young brains,” said Dr. Roneet Lev, an emergency room physician at Scripps Mercy Hospital in San Diego. 

Jahan Marcu, scientific adviser for the Coalition for Cannabis Scheduling Reform, which represents cannabis companies, said research on connections between psychosis and cannabis doesn’t sufficiently distinguish between different kinds of products.

“Any time we talk about any substance, it’s just a factor. It can be a good factor, and it can be a bad factor,” he said.

Dr. Karen Randall moved to Pueblo, Colo., for a job in an emergency room more than a decade ago after working in Detroit for 18 years. She thought it would be like retiring early. She bought a ranch where she could ride horses in an area locals now call the Napa Valley of cannabis. 

“I see more psychotic people here than I did in Detroit,” she said. “We’re just making this huge population of people who we can no longer fix.” 

She is considering quitting medicine.

Randy Bacchus started smoking marijuana as a freshman in high school in Mahtomedi, Minn. By the time he was old enough to move out, his life had spun out of control.

In one of hundreds of videos his parents found on his phone after he died, Randy Bacchus told would-be fans that he was going to be a rap star. The 21-year-old held a vape pen in his hand, stopping to take hits at intervals.

He regularly talked about smoking cannabis in his videos. His interactions with his parents had been increasingly angry and erratic. He claimed he had spoken to God and the devil.

One night after using cannabis he had been so afraid people were after him that he ran out into a snowstorm in Denver and was lost for 24 hours. He survived with frostbite and infected toes.

“I was in full-blown psychosis,” he said in another video.

His parents called the police for a wellness check but said that because he was an adult and not a danger to himself or others, they couldn’t force him into treatment. 

“I think I’m going to take a break from smoking for a sec because I’m enjoying it too much,” he said in a March 2021 video. 

In July 2021, he texted his mom to say he wanted to stop using cannabis and give up on music. 

“I love you and am sorry for everything. I love dad and the same to him. I wish I would have been a better person,” he wrote at 2:09 a.m.

His mother wrote back that life isn’t easy and it is never too late. Today is a fresh start, she said. Police found him dead 48 hours later from a self-inflicted gunshot wound.

Doctors who treated Randy said his cannabis use was triggering psychotic episodes. In the videos on his phone, he said he believed cannabis had caused his delusions. 

“I didn’t know that marijuana could cause paranoia,” said Heather Bacchus, Randy’s mom. “They don’t even know what they’re smoking.”

Write to Julie Wernau at julie.wernau@wsj.com