Promethazine has little to no recreational value, however it's a pretty solid anti-histamine. Which are HIGHLY recommended to take with opiates, to combat the nausea.
Which is why it's commonly found in prescription opiates such as codeine syrups. At pharmaceutical doses though, overdosing anti-histamines is never bright(anti-histamine poisoning for one).
Taking more than 50 mgs of promethazine within 24 hours may cause severe problems.