fentanyl, cyproheptadine. Possibly will increase toxicity in the other by pharmacodynamic synergism. Modify Therapy/Check Closely. Coadministration of fentanyl with anticholinergics might improve risk for urinary retention and/or serious constipation, which may bring about paralytic ileus.If coadministration of CYP3A4 inhibitors with fentanyl is cr