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. IT IS METABOLISED BY CYP-450 ISOENZYME 3A4, THEREFORE USE WITH OTHER DRUGS WHICH INDUCE OR INHIBIT THIS ENZYME MAY RESULT IN ALTERED PLASMA CONCENTRATION OF THIS DRUG.