shit 25mg promethazine = 100mg diphenhydramine, or is as sedative as 100mg diphenhydramine or 75mg hydroxyzine.
if you have a experience taking sedating (first-gen) antihistamines or have taken a TCA (most TCAs have strong affinity for histamine receptors), then you could get away with 75-100mg promethazine. if you dont pass out after taking 75-100mg promethazine i will get your mad props. also, with promethazine being so much more sedating than DPH, you will probably pass out before hallucinations.
but like i said i can take 150mg hydroxyzine with my bupre and valium and be fine, but i have taken hydroxyzine and DPH for sleep for 4+ years, and have taken every day for the past 9+ months and i would probably be K.O.'d at 50-75mg of promethazine.