C.N.S. DEPRESSANTS SUCH AS BENZODIAZEPINES, BARBITURATES, TRANQUILLIZERS AND GENERAL ANAESTHETICS MAY HAVE ADDITIVE OR POTENTIATING EFFECTS. ATRACURIUM AND VECURONIUM MAY RESULT IN SEVERE BRADYCARDIA. INCREASED RISK OF RESPIRATORY DEPRESSION, AND HYPOTENSION WITH HALOTHANE. INCREASED ANTICOAGULANT EFFECT. RESPIRATORY DEPRESSANT EFFECT ENHANCED BY LIDOCAINE. ACTION POTENTIATED BY ALCOHOL, CNS DEPRESSANTS,BENZODIAZEPINES, ANAESTHETICS, HYPNOTICS, TRICYCLIC ANTIDEPRESSANTS,PHENOTHIAZINES & ANTIPSYCHOTICS. INCOMPATIBLE WITH SOLUBLE BARTITURATES. PATIENTS PREVIOUSLY GIVEN NARCOTICS FOR THE TREATMENT OF NARCOTIC DEPENDENCE , HAVE EXPERIENCED WITHDRAWAL SYMPTOMS AFTER RECIEVING PENTAZOCINE. CYCLIZINE MAY COUNTERACT THE HAEMODYNAMIC BENEFITS OF OPIOIDS. CIMETIDINE BUT NOT RANITIDINE INHIBITS THE METABOLISM OF PETHIDINE AND SOME OTHER OPIOIDS. OPIOIDS GASTROINTESTINAL EFFECTS MAY DELAY ABSORPTION OF SOME DRUGS AND MAY BE COUNTERACTIVE WITH CISAPRIDE, DOMPERIDON OR METOCLOPRAMIDE. MAY CAUSE SEVERE AND SOMETIMES FATAL SIDEEFFECTS WHEN GIVEN IN PATIENTS WHO ARE ON MAOIS. AN INCREASE IN AUC AND IN ELIMINATION HALF-LIFE HAS BEEN REPORTED IN SUBJECTS GIVEN FENTANYL WITH RITONAVIR.