Drowsiness, dizziness, constipation, or headache may occur. If any of these effects persist or worsen, tell your doctor or pharmacist immediately. To prevent constipation, eat a diet adequate in fiber, drink plenty of water, and exercise. Consult your pharmacist for help in selecting a laxative (such as a stimulant type with stool softener). 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 narcotics). Tell your doctor immediately if any of these unlikely but serious side effects occur: slow/shallow breathing, fainting, mental/mood changes (such as agitation, confusion, hallucinations), stomach/abdominal pain, unusual drowsiness/difficulty waking up. Although this medication is used to prevent withdrawal reactions, it may rarely cause narcotic 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 narcotics such as methadone. If such symptoms occur, notify your doctor or pharmacist immediately. This drug may rarely cause serious liver disease. Get medical help right away if you have any symptoms of liver damage, including: dark urine, persistent nausea/vomiting, yellowing eyes/skin, severe stomach/abdominal pain. A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.