How Do Marijuana and Alcohol Interact Inside Your Brain?

Crossfading with two of America’s favorite mood-altering drugs.

Posted Dec 14, 2019

Two of America’s favorite mood-altering drugs, marijuana and alcohol, are being consumed together, called crossfading, more frequently as marijuana becomes less regulated. Thus, there is a good reason to try to understand what is going on inside the brain when these drugs are taken together.

You might expect that the answer to this question would already be well known; after all, anthropologists claim that humans have been using both alcohol and marijuana for at least 12,000 years. Scientists still need a little more time to catch up.

The problem is that no one knows how alcohol will interact with all of the compounds found in marijuana. Also, there are a number of other important considerations. Was the marijuana smoked or eaten? The route of administration affects how much THC gets into the blood and peak blood levels. Does alcohol influence how much THC, the principal psychoactive ingredient of marijuana, gets into the blood and brain?

A study published in the Journal of Analytical Toxicology (2013, Apr;37(3):152-8) reported that consuming alcohol (either a little or a lot) prior to smoking marijuana did not influence how much THC was found in the blood. Thus, these two drugs do not appear to directly influence each other’s access to the blood and brain. 

Alcohol and marijuana have vastly different actions within the brain. This makes predicting how the combination might affect mood or thinking exceptionally difficult. For example, alcohol is considered a brain depressant, and it is possible to kill yourself with alcohol.

In contrast, marijuana is neither a stimulant nor a depressant in the brain, and it is not possible to kill yourself with marijuana. Both drugs act virtually everywhere inside the brain, and both drugs produce euphoria by enhancing the actions of the neurotransmitter dopamine. However, marijuana has important psychoactive actions that do not depend upon dopamine. 

So, what can you expect to happen when using both drugs together? There are numerous testimonials on the internet claiming that the combination made the experience both better and worse than using either drug alone. These contradictory reports actually make sense.

Scientists have known for many decades that combining psychoactive drugs can produce highly variable effects upon brain function. Obviously, this is true for marijuana and alcohol. Neuroscientists have shown that alcohol requires the action of the neurotransmitter GABA in order to produce its effects on mood and thinking ability. In contrast, marijuana prevents GABA from being released inside the brain; this action would tend to reduce the effects of alcohol.

Alcohol and marijuana also affect the generation of an important brain wave called gamma. You might have heard of alpha and delta waves; gamma waves are also a type of brain wave that has a much higher frequency, i.e., from 30 to 200 Hz.

Gamma waves are distributed widely throughout the brain, and they participate in various functions, such as perception, attention, memory, and consciousness. Alcohol disrupts gamma waves by enhancing GABA, while marijuana disrupts gamma waves by inhibiting GABA. Currently, no one can predict what these vastly differential effects on gamma waves might feel like inside your head.

Obviously, there is still too little specific information available in the scientific literature to predict what will happen when both of these drugs are present in the brain. My prediction is that marijuana will tend to reduce the effects of alcohol. Sadly, due to a total lack of knowledge, combining marijuana and alcohol is still a pharmacological roulette.

© Gary L. Wenk, Ph.D., is the author of Your Brain on Food, 3rd Edition, 2019 (Oxford University Press).