fentanyl, cyproheptadine. Either increases toxicity on the other by pharmacodynamic synergism. Modify Therapy/Check Carefully. Coadministration of fentanyl with anticholinergics could boost risk for urinary retention and/or intense constipation, which can result in paralytic ileus.If coadministration of CYP3A4 inhibitors with fentanyl is critical,