What is so bad about drinking alcohol while on promethazine dm? no additional details needed

Alcohol : Alcohol can worsen some of the side effects seen with promethazine. Both alcohol and Promethazine are considered central nervous system depressants. The two together can not only cause psychomotor impairment (slowing of thought and movement) but can also cause a dystonic reaction which is a neurological movement disorder characterized by muscle twitching or uncontrollable movements. Because of these reactions, alcohol should be avoided while taking promethazine.

Drug interactions: Would need to know why you are on Promethazine for give any kind of rational answer, but alcohol increases the activity of Promethazine and vice versa; in other words there is more toxicity with both drugs together than just adding up the toxicity of the two separately. Driving on that combination would be particularly dangerous. How much alcohol? A little, not much; a lot, a bunch of problems.