Decreased resp reserve or those receiving other CNS depressant drugs; presence of phenobarb or any other enzyme-inducing agents. Administration during labour; supraventricular tachycardia; head injuries, biliary tract disorders, hypothyroidism, adrenocortical insuficiency, shock, prostatic hypertrophy; renal failure, cancer or sickle anaemia. Cross-tolerance between narcotic analgesics can occur. May affect ability to drive or operate machinery. Hepatic disease. Severe renal impairment. Newborn, premature infants or childn <1 yr. Elderly & debilitated.