Nausea, vomiting, drowsiness, dizziness, constipation, or headache may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly.
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.
Although this medication is used to prevent withdrawal reactions, it may rarely cause opioid withdrawal symptoms (see also How to Use section). This is more likely to happen when you first start treatment or if you have been using long-acting opioids such as methadone. If such symptoms occur, tell your doctor or pharmacist right away.
Severe (possibly fatal) breathing problems can occur, especially 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), signs of your adrenal glands not working well (such as unusual tiredness, weight loss).
Get medical help right away if you have any very serious side effects, including: fainting, fast/irregular heartbeat, severe dizziness, mental/mood changes (such as agitation, confusion, hallucinations), slow/shallow breathing, unusual drowsiness/difficulty waking up.
This drug may rarely cause serious (possibly fatal) liver disease. Get medical help right away if you have any symptoms of liver damage, including: dark urine, persistent nausea/vomiting/loss of appetite, severe stomach/abdominal pain, yellowing eyes/skin.
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 or at www.fda.gov/medwatch.
In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.