The doctors told Regina Denney and her son Brian Smith Jr. what was causing his severe vomiting and abdominal pain.

Neither the teenager nor his mother believed what they said: smoking weed.

Smoking marijuana, the two knew, was recommended to cancer patients to spur the appetite. How could it lead to Brian’s condition?

As the months went by and the pounds slipped off Brian’s once healthy frame, it was clear that whatever was causing his stomach troubles had just the opposite effect.

Brian kept smoking. The symptoms continued on and off.

Last October, after another severe bout of vomiting, the teenager died. He was 17 years old.

Five months later, as Denney pored over a coroner’s report for answers, she finally accepted that marijuana played a pivotal role in her son’s death. The autopsy report, which Denney received in March, attributed her son’s death to dehydration due to cannabinoid hyperemesis syndrome.

A week before he died, Brian Smith Jr., right, enjoyed time with his nephew Zayden. Brian died in 2018 from dehydration after excessive vomiting. The vomiting was a result of a rare condition called cannabinoid hyperemesis syndrome.

“We had never heard about this, had never heard about marijuana causing any vomiting. He and I were like, ‘Yeah, I think it’s something else,’ ” Denney said. “Brian did not believe that was what it was because of everything we had ever been told about marijuana. … It didn’t make any sense.”

Cannabinoid hyperemesis syndrome, also known as CHS, can arise in response to long-term cannabis use. The syndrome consists of vomiting, nausea and abdominal pain, which can often be alleviated by taking hot showers.

Doctors say CHS is on the rise, but they are not certain why. Marijuana is more available than in years past, and it is more potent.