It's metabolized by CYP2D6, so if the affinity of promethazine to CYP2D6 is higher than the affinity of another substance to CYP2D6, it will inhibit the metabolism of the other drug
Definitely inhibitor. 2D6 is the most resistant to induction, with many articles saying there are no known inducers. There are a few, but promethazine isnt one.