Let's start with what you probably already know: Alcohol is a depressant, but in low doses it causes emotional release and lowers inhibitions. Marijuana is also known for its relaxing qualities, but can produce very different results depending on how much and what strain of it you smoke. So what happens when you mix them together? One study looked at how smoking weed affects the absorption of alcohol, and the other looked at how drinking alcohol affects the absorption of THC. According to Scott Lukas, a professor of psychiatry and pharmacology at Harvard Medical School, found smoking cannabis activates your body's cannabinoid 2 receptors (CB2), which can affect how quickly your body absorbs alcohol.
"Marijuana does a unique thing to your small intestine that alters the motility [the way things move through your intestines] of your GI tract in such a way that it causes your blood alcohol levels to actually be lower than…if you had just consumed alcohol by itself," Lukas says. But in the second study, Lukas found that alcohol actually has the inverse effect on THC: If you drink first and then smoke, it causes the levels of THC in your plasma to skyrocket, intensifying your high. That's because alcohol opens up blood vessels in your digestive system, which helps THC get absorbed—a finding confirmed in a more recent study done in 2015.
As most recreational marijuana users can attest, however, there are limits to this feel-good effect: Drink too much before you smoke, and you run the risk of "greening out"—a nauseous sensation that kicks in when you feel sick and overwhelmed after getting too high. (Trust me, it's no fun.) "Individuals may go pale and sweaty, feel dizzy with 'the spins,' nauseous, and may even start vomiting. This is often followed by the need or strong desire to lie down," wrote Constance Scharff, an addiction specialist in California, in a column for Psychology Today.