(NewsNation) — As the legalization of recreational marijuana spreads, a growing body of research suggests there could be a possible risk between the drug and serious mental health issues.

There is a concern about a possible link between cannabis, psychosis and schizophrenia, and the research suggests those at the highest risk are young men.

Laura and John Stack are still coming to terms with the suicide of their son in 2019. The Stacks live in Colorado, the first state to legalize marijuana in 2012. They say their son, Johnny, became a regular user in his teens.

“He would say, ‘What do you want mom and dad, I have a 4.0.’ And it was kind of hard to argue with that. And he didn’t seem to have any problems until he did,” Laura said.

Compounding his problems, Johnny was also using pot products made with extracts that produce no tell-tale smell.

“He had started vaping. And it was completely odorless. And from about the age of 14 through 1, we had no idea at all,” John said.

By 18, Johnny was a heavy user and exhibiting symptoms of psychosis.

“He started to think that people were listening to him through his iPhone, he would buy burner phones and put sticky notes over the webcam,” Laura said. “He was saying that people were watching him, he began to think that the mob was after him.”

The Stacks are now left to cherish photos and videos, including one of Johnny reciting a poem three days before he took his own life.

Since the spread of legalization over the past decade, a number of studies have looked into the health impacts of marijuana.

The latest is a Danish study co-authored by a director at the National Institute of Health.

“If it does produce psychosis and acute psychotic episodes, that can leads (to symptoms) that can be very, very terrifying. Someone could basically kill themselves in a very impulsive act,” Dr. Nora Volkow said.

In the study, published this month in “Psychological Medicine,” researchers said there’s enough data to confirm an “association between cannabis use disorder and schizophrenia is stronger in young males than females.

“That period of transition between the pre-teens and the 20s, is exactly when you start to see the emergence of schizophrenia but the risks are significantly higher for males than females,” Volkow said.

Marijuana legalization has ushered in a new generation of potent pot, which contains much more THC, the part of the plant that induces a high.

Back in the 1970s, the average plant had a THC level under 3%. Genetic engineering over the decades has pushed that average up to nearly 25%.

Dr. Christine Miller is a neurobiologist and psychosis expert.

“They’ve done studies in Europe where they’ve administered purified THC to subjects in a clinical study and they found that 40% of those with no family history of psychosis developed psychotic symptoms,” she said.

Recreational sales are now legal in 22 states, along with Washington, D.C. Only Vermont and Connecticut have caps on potency.

Three days before Johnny died, the Stacks said he told them he was sorry and he loved them, but marijuana ruined his mind and his life.

The Stacks are now traveling the country to warn others about the dangers of potent pot. Earlier this month, Oklahoma joined a number of other states in rejecting legalization for recreational use.