movie of man that turns into a wolf or some kind of monster with long nail in a dressing room

I remember waking up in the middle of the night when I was like 6 and seeing this movie playing in the tv it was so scary that I remember it after 15+ years. The movie is 80s style and I remember there was 2 guys and a girl (I think one of the guys was dating the girl) in the movie one of the guys turns into a wolf or some kind of monster with long nail in a dressing room or a bathroom with stalls. After that there’s another scene that I remember where this transformed creature is walking down some stairs and there’s blood and fire everywhere.
I can’t remember anything else… please help me find it!

thank you for your help!