As the “California sober” trend of ditching alcohol in favor of cannabis gains momentum, a new study on the causal effect of cannabis on alcohol consumption suggests that smoking marijuana may lead people to drink less in the short term.

Published in the American Journal of Psychiatry, the study by researchers at Brown is the first randomized, placebo-controlled trial to test whether marijuana use directly changes alcohol consumption. Unlike previous research that relied on surveys or self-reported associations, this trial allowed scientists to measure cause and effect under controlled laboratory conditions.

“Instead of seeing cannabis increase craving and drinking, we saw the opposite,” says Jane Metrik, PhD, a professor of behavioral and social sciences and of psychiatry and human behavior. “Cannabis reduced the urge for alcohol in the moment, lowered how much alcohol people consumed over a two-hour period, and even delayed when they started drinking once the alcohol was available.”

Excessive drinking is the third leading cause of preventable death in the United States. Cannabis use often overlaps with alcohol problems, with about 60 percent of people with cannabis use disorder also meeting criteria for alcohol use disorder.

The trial included 157 adults, ages 21 to 44, who drink heavily and use cannabis at least twice a week. Over three separate lab visits, each participant smoked cannabis cigarettes containing lower or higher levels of THC, the main psychoactive ingredient in marijuana, or a placebo. Then, in a room designed to resemble a bar, they were offered servings of their preferred alcoholic beverage and could choose either to drink or to earn small cash payments for each drink they declined.

The researchers found that participants who smoked the cannabis with THC drank less alcohol than when they smoked the placebo. They also reported less immediate urges to drink than when they smoked a placebo cigarette.

“We saw that cannabis reduces the urge in the moment,” Metrik says. “What we don't know from this study is what is the long-term effect.”

The researchers caution that the results of the study don’t mean that cannabis should be recommended as a therapeutic substitute for alcohol. They also noted that it remains unclear whether real-world settings, where people drink socially or use higher-potency cannabis, would yield similar results. More study is warranted before cannabis can be considered a tool to help people cut back or stop drinking altogether.

“Our job as researchers is to continue to answer these questions,” Metrik says. “We can’t tell anyone yet, ‘you should use cannabis as a substitute for problematic or heavy drinking.’”