Both the effects of marijuana and alcohol have on the human brain have been widely studied, but the same thing can’t be said about the combination of the two, which is rather odd considering a lot of people enjoy a drink or two while packing a bowl. Scott Lukas, a professor at Harvard Medical School, investigated what happens in the brain while cross-faded in 2011 and came to some surprising conclusions.

First off, it’s important to note that marijuana (THC to be more specific) and alcohol are two psychoactive substances that are far from being similar to one another. THC acts upon the brain’s cannabinoid receptors, sparking intense cognitive effects like paranoia, a distorted sense of time, attention deficit and more. Alcohol, on the other hand, depresses the nervous system with significant consequences to motor skills – walking in a straight line can be the arduous quest while in the drunken haze.

So, one might think that if you smoke and drink at the same time, the two effects will combine. Neurochemistry is far more complicated than adding left and right, though. Lukas found that not only did the two effects combine, but in some instances, the effects considerably became amplified. For instance, Lukas noticed that people who smoked a joint and also drank a lot of alcohol (a couple of shots) had twice as much THC in their blood than those who didn’t drink at all. Oddly, this seems to happen only when you drink first, and then smoke.

This happens, Lukas says, because alcohol opens up blood vessels in the gastrointestinal tract helping the THC to be absorbed more efficiently.

Also, those who smoked and drank self-reported the high effects much sooner and rated their high as ‘better’. However, this works only to a point. If you drink too much before lighting the first joint, you risk getting ‘greened out’. That is, suffering from nausea, bouts of vomiting, and an intense urge to lie down.

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Oddly enough, in a second study run by Lukas and colleagues, the researchers found smoking first followed by drinking results in less alcohol in the bloodstream. THC seems to alter the motility of the gastrointestinal tract in such a way that it lowers alcohol levels.

While all this may sound pretty impressive, it’s important to note that combining alcohol and marijuana enhances not only the high but also the lows. Impaired judgment and increased heart rate – usual symptoms while high on weed – are stronger. It’s also worth being aware that marijuana has an antiemetic effect, meaning it makes it harder for the body to vomit. As you likely know, vomiting is the body’s first line of defense against severe alcohol intoxication. In extreme cases, a user might choke on the vomit and die. Combining weed and alcohol could also be potentially dangerous for people with heart problems and also increases the risk of accidents.

Bottom line: alcohol and marijuana can mix well but only if you’re very sensible about the quantities you ingest. Like anything, don’t go overboard.