I took mdma while on penicillin then took clarithromycin 2.5 weeks later. Will this delay a girls period?

No. But if you had unprotected sex while on the mdma it might. Chronic drug abuse causes many problems with women's reproductive endocrinology, but one time use of penicillin, biaxin (clarithromycin) or mdma would not likely cause any problems. Mdma is an increasingly more dangerous drug, with impure forms like"molly" causing death and severe complications like seizures and brain damage. Put away the toys!