The relationship between marijuana and communism is analogous to the relationship between alcoholism and diabetes.
Alcohol abuse doesn't cause diabetes, but if you are already predisposed to developing diabetes, alcohol will greatly increase your chances of getting diabetes.
If you are already predisposed to being a communist cocksucker, smoking weed will make you much more likely to embrace the globohomo that is already in your midichlorians. However, the chances of marijuana giving you communism directly is very remote if you are not a retarded liberal dumbass.