Like sekio mentioned, the probability of you dying or injuring yourself is strongly dependent on the respective doses of the drugs suggested, the size and weight of the intended taker, and the degree of preexisting tolerance the taker has to each of them. As a general rule, I would say that mixing a benzo with two phenothiazines and a beta blocker is not a good idea (and probably won't properly get you high, either). It sounds like the sort of cocktail that a terrified newbie at a mental hospital would inject into someone who's violently psychotic). Prochlorperazine is a potent antipsychotic (dopamine D2 antagonist, which is no fun to take in the short term and in the long term has some pretty ugly side effects), promethazine is a really dirty drug (antagonizes damn near everything, including histamine, acetylcholine, dopamine, and a lot of serotonin pathways), and nonselective beta blockers are OK anxiolytics but also drop your blood pressure in most cases. The thing I'm curious about is why your friend (or whoever you're asking this on behalf of) wants to take all of these things. For recreational purposes none of these are really great, but the Valium should be a nice downer. If it's as a sleep aid, stick to either the Valium (or try a z-drug like Ambien) or the promethazine, and add melatonin to whichever one of those you choose before mixing them both together. If it's anxiolysis you're after (and you have normal to high blood pressure), then try propanolol first in low doses, then go to the Valium if needed and get advice from a doctor before mixing the two. If it's nausea you're having a problem with, see if you can get a scrip for Zofran before messing with the phenothiazines (and if you must use a phenothiazine start with promethazine before turning to prochlorperazine).