Nausea, headache, dizziness, drowsiness, anxiety, tiredness, and loss of appetite may occur. If you have been using opiate narcotics regularly, mild opiate withdrawal symptoms may occur, including abdominal cramps, restlessness, bone/joint pain, muscle aches, and runny nose. If any of these effects persist or worsen, tell your doctor or pharmacist promptly. Pain/redness/swelling/itching/bruising at the injection site may also occur. If any of these effects steadily worsen, or if they persist longer than two weeks, tell your doctor or pharmacist immediately. Rarely, a severe injection site reaction can cause permanent injury if not treated. 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. Sudden opiate withdrawal symptoms can occur within minutes after using naltrexone if you are physically addicted to narcotics. Tell your doctor immediately if any of these withdrawal symptoms occur: vomiting, diarrhea, mental/mood changes (e.g., anxiety, confusion, extreme sleepiness, visual hallucinations). Tell your doctor right away if you have any serious side effects, including: fast/irregular heartbeat, depression/rare thoughts of suicide, signs of a serious breathing problem/pneumonia (e.g., cough, shortness of breath, wheezing), blisters/sores at injection site. Get medical help right away if this rare but very serious side effect occurs: chest pain. Naltrexone has rarely caused serious liver disease. The risk is increased when larger doses are used. (See also Warning section.) 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.