Cannabis and Alcohol: What Actually Happens When You Mix Them

Combining cannabis and alcohol — colloquially known as "crossfading" — is one of the most common drug combinations in the world. It is also one of the most pharmacologically complex, with effects that depend heavily on the order of consumption, the doses of each substance, and individual factors including body weight, tolerance, and genetics. Understanding the science can help you make safer choices.

The most important finding from research on cannabis-alcohol interactions is that the order matters enormously. When alcohol is consumed before cannabis, blood THC levels are significantly higher than when cannabis is consumed alone. A 2001 study published in Drug and Alcohol Dependence found that alcohol consumption increased peak plasma THC concentrations by up to 56%. The mechanism is straightforward: alcohol increases the permeability of the gastrointestinal tract, accelerating THC absorption.

This means that drinking before using cannabis — even a small amount of alcohol — can dramatically amplify the effects of cannabis. What would normally be a mild cannabis experience can become overwhelming. This is the pharmacological explanation for the "greening out" phenomenon that many people have experienced: consuming what seemed like a normal amount of cannabis after drinking and experiencing intense nausea, dizziness, and anxiety.

The reverse combination — cannabis before alcohol — has different effects. Research suggests that cannabis may slow gastric emptying, which can reduce the rate of alcohol absorption and potentially lower peak blood alcohol concentration. Some studies have found that people who use cannabis before drinking consume less alcohol overall, possibly because cannabis reduces the desire to drink more. However, this does not mean the combination is safe — it means the risks are different.

Both substances impair driving, and their combination is more dangerous than either alone. A meta-analysis of crash risk studies found that cannabis use approximately doubles crash risk, alcohol use at legal limits approximately doubles crash risk, and the combination of both increases crash risk by a factor of 7 to 16. This is not additive — it is synergistic. The two substances impair different aspects of driving ability, and together they impair nearly all of them.

Nausea and vomiting are the most common adverse effects of combining cannabis and alcohol. The mechanism involves both substances acting on the same brainstem nausea centers through different pathways. Cannabis normally has antiemetic (anti-nausea) properties, but at high doses combined with alcohol, this effect can reverse. If you feel nauseous after combining the two substances, lying down with your eyes closed and staying hydrated is the safest approach.

Long-term, the combination of heavy cannabis and alcohol use is associated with greater cognitive impairment than either substance alone. Studies of young adults who use both substances heavily show deficits in memory, executive function, and processing speed that are greater than the sum of the individual effects. This is particularly concerning for adolescents, whose brains are still developing.

Harm reduction principles for those who choose to combine: start with very low doses of both substances; consume cannabis after alcohol rather than before; avoid high-THC concentrates when drinking; stay hydrated; have a sober person present; and never drive. If you experience severe anxiety, rapid heart rate, or feel like you might lose consciousness, seek medical attention — while cannabis alone cannot cause a fatal overdose, the combination with alcohol can cause dangerous levels of impairment.