Why can't you mix codine with alcohol?

You can, but it's not recommended. The reason for this is, codeine is a narcotic and alcohol is, well, alcohol. The alcohol will increase the effects of the codeine. But also don't forget that codeine is generally composed with Acetaminophen (Tylenol) which is hard on your liver, and so is alcohol of course. Prolonged use of either or both can cause many liver problems such as: an enlarged liver, liver spots, cirrhosis of the liver, and(or) hepatitis.