First, the basics: Marijuana contains THC (tetrahydrocannabinol), which acts on the brain's cannabinoid receptors. Alcohol depresses the central nervous system. Trying to compare the two isn't even like comparing apples and oranges, says Gary Wenk, a professor of psychology and neuroscience at Ohio State University. "It's apples and vegetables. They're very different drugs." An extremely simplified explanation would be to say that THC largely has cognitive effects, like paranoia and a distorted sense of time, while alcohol mainly affects motor skills, making it hard to walk in a straight line and causing slurred speech.