However, severe reactions such as respiratory depressions and pronounced hypotension are uncommon when the drug is used therapeutically. Drop in blood pressure due to a cardiac infarction or hypovolemia can have dangerous results.
Constipation is the most important problem in long-term treatment.
Somnolence and nausea are often only initially disturbing. Even therapeutical doses can reduce respiratory frequency and can cause oliguria or urinary retention. Further potential side effects are euphoria and dysphoria, anxiety, pruritus, urticaria, bronchospasms, spasms of the sphincter oddi, edemas, headaches, miosis. Development of tolerance entails ever-increasing doses.