fentanyl, cyproheptadine. Both raises toxicity from the other by pharmacodynamic synergism. Modify Therapy/Watch Carefully. Coadministration of fentanyl with anticholinergics may well increase risk for urinary retention and/or critical constipation, which can cause paralytic ileus.If coadministration of CYP3A4 inhibitors with fentanyl is necessary,