Drowsiness, dizziness, constipation, or headache may occur. If any of these effects persist or worsen, tell your doctor or pharmacist right away. To prevent constipation, eat dietary fiber, drink enough water, and exercise. You may also need to take a laxative. Ask your pharmacist which type of laxative is right for you. To reduce the risk of dizziness and lightheadedness, get up slowly when rising from a sitting or lying position. Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

Severe (possibly fatal) breathing problems can occur if this medication is abused, injected, or mixed with other depressants (such as alcohol, benzodiazepines including diazepam, other opioids). Tell your doctor right away if you have any serious side effects, including interrupted breathing during sleep (sleep apnea), mental/mood changes (such as agitation, confusion, hallucinations), stomach/abdominal pain, signs of your adrenal glands not working well (such as unusual tiredness, weight loss). Get medical help right away if any of these rare but serious side effects occur: fainting, fast/irregular heartbeat, severe dizziness, slow/shallow breathing, unusual drowsiness/difficulty waking up.

Although this medication is used to prevent withdrawal reactions, it may rarely cause opioid withdrawal symptoms, including diarrhea, severe mental/mood changes (such as anxiety, irritability, trouble sleeping), muscle stiffness, or shakiness. This is more likely when you first start treatment or if you have been using long-acting opioids such as methadone. If such symptoms occur, notify your doctor or pharmacist right away.