By the time he reached college, he had been through various addiction treatment programs. He had become so paranoid that he thought the mob was after him and his college was a base for the F.B.I., Ms. Stack said. At one point, after he moved out of his childhood home, he threatened to kill the family dog unless his parents gave him money. His mother later discovered that Johnny had obtained his own medical marijuana card when he turned 18 and had begun dealing to younger kids.

After several stays at mental hospitals, the doctors determined that Johnny had a severe case of THC abuse, Ms. Stack said. He was prescribed an anti-psychotic medication, which helped — but then he stopped taking it. In 2019, Johnny died after jumping from a six-story building. He was 19. A few days before his death, Ms. Stack said, Johnny had apologized to her, saying that weed had ruined his mind and his life, adding, “I’m sorry, and I love you.”

A recent study found that people who used marijuana had a greater likelihood of suicidal ideation, plan and attempt than those who did not use the drug at all. Ms. Stack now runs a nonprofit called Johnny’s Ambassadors that educates communities about high-THC cannabis and its effect on the adolescent brain.

There’s ‘no known safe limit.’

It can be difficult to pinpoint exactly how much THC enters someone’s brain when they’re using cannabis. That’s because it’s not just the frequency of use and THC concentration that affect dosage, it’s also how fast the chemicals are delivered to the brain. In vaporizers, the speed of delivery can change depending on the base the THC is dissolved in, the strength of the device’s battery and how warm the product becomes when it’s heated up.

Higher doses of THC are more likely to produce anxiety, agitation, paranoia and psychosis.

“The younger you are, the more vulnerable your brain is to developing these problems,” Dr. Levy said.

Youths are also more likely to become addicted when they start using marijuana before the age of 18, according to the Substance Abuse and Mental Health Services Administration.