New research shows heavy cannabis use may affect motivation, but it doesn’t increase a person’s likelihood of ‘reefer madness’ like scientists once thought.

Pot smokers are stereotyped as unmotivated slackers, and new research says that regular marijuana use may have that effect.

Long-term marijuana use appears to have an effect on dopamine levels in the brain’s striatum, which research has shown is linked to novelty-related decision making and motivation. There have also been theories circulated for years that marijuana use increases a person’s likelihood of developing a mental disorder like schizophrenia because of the effect of marijuana on dopamine levels in the brain.

Researchers at Imperial College London, University College London, and King’s College London say their study “could explain why some cannabis users appear to lack motivation to work or pursue their normal interests.”

To come to their conclusion, the researchers performed positron emission tomography (PET) scans on the brains of 38 similar people with one big difference—19 of them “experienced psychotic-like symptoms when they consumed cannabis” while the other 19 didn’t smoke marijuana at all.

The cannabis users weren’t weekend warriors—they all used marijuana heavily enough to meet the criteria for abuse or dependence and began smoking at an early age, some as young as 12.

Lead study author Dr. Michael Bloomfield of the Institute of Clinical Sciences at Imperial College London said that, though their research only involved pot smokers who experienced “psychotic-like experiences,” he believes, somehow, that the dopamine level changes observed in the subjects’ brains could be found in everyone who smokes pot.

“It could also explain the ‘amotivational syndrome’ which has been described in cannabis users, but whether such a syndrome exists is controversial,” Bloomfield said in a press release.

Researchers found that high levels of cannabis use were directly associated with dopamine synthesis in the striatum, but not associated with cannabis-induced psychotic symptoms.

Bloomfield said the results weren’t what researchers were expecting, but they tied in with previous work on addiction, “which has found that substance abusers—people who are dependent on cocaine or amphetamine, for example—have altered dopamine systems.”

“These findings indicate that chronic cannabis use is associated with reduced dopamine synthesis capacity and question the hypothesis that cannabis increases the risk of psychotic disorders by inducing the same dopaminergic alterations seen in schizophrenia,” researchers concluded in their study, published in the journal Biological Psychiatry.

But there’s hope for people who can quote Half Baked verbatim: while previous research suggested that chronic (no pun intended) marijuana smokers are more likely to develop schizophrenia, further research is showing that may not be the case.

Earlier this year, researchers had 2,120 teens ages 16 to 19 report on their marijuana use and mental state, including any thought, social, and attention problems. That research, published in the journal Addiction, suggests that marijuana use and altered states of mind go hand in hand.

“Cannabis use predicts psychosis vulnerability in adolescents and vice versa, which suggests that there is a bidirectional causal association between the two,” researchers concluded in that study.

It should be noted that psychosis is a separation from reality, and marijuana is a hallucinogenic drug, so the chicken-and-egg argument will require more research to sort out.

The plant’s psychoactive chemical tetrahydrocannabinol (THC) can produce feelings of paranoia when consumed in large amounts. However, marijuana, alcohol, and other mind-altering substances are often used by the mentally ill as a form of self-medication.

One of the appeals of pot—especially in medical marijuana circles—is its relaxing effect on conditions like anxiety and depression. When it comes to affecting a person’s motivation, however, the jury is still out.

In the movie Jackie Brown, Samuel L. Jackson warns Bridget Fonda’s character that smoking too much pot will rob her of her ambition, but she’s none too worried about that.

“Not if your ambition is to get high and watch TV,” she replies.

History has shown that many successful, ambitious people can smoke pot, including the past three Presidents of the United States, a sitting Supreme Court justice, and enough celebrities to fill a NORML conference.