See also How to Use section. Dizziness, trouble sleeping, drowsiness, unusual dreams, and trouble concentrating may frequently occur. These side effects may begin 1-2 days after starting this medication and usually go away in 2-4 weeks. They are also reduced by taking efavirenz on an empty stomach at bedtime. Avoid alcohol and street drugs while taking efavirenz. They may worsen these side effects. Tiredness, headache, nausea, vomiting, and diarrhea may also occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly. 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. Infrequently, serious psychiatric symptoms may occur during efavirenz treatment, although it is unclear if they are caused by efavirenz. These effects may be seen especially in people who have mental/mood conditions. Tell your doctor immediately if any of these unlikely but serious side effects occur: mental/mood changes (such as depression, rare thoughts of suicide, nervousness, angry behavior, hallucinations). Tell your doctor immediately if any of these rare but serious side effects occur: signs of liver problems (such as persistent nausea/vomiting, stomach/abdominal pain, severe tiredness, yellowing eyes/skin, dark urine). Changes in body fat may occur while you are taking this medication (such as increased fat in the upper back and stomach areas, decreased fat in the arms and legs). The cause and long-term effects of these changes are unknown. Discuss the risks and benefits of treatment with your doctor, as well as the possible use of exercise to reduce this side effect. Efavirenz can commonly cause a rash that is usually not serious. A rash may occur in the first 2 weeks after starting treatment and if it is not serious, it will usually resolve in 4 weeks. However, you may not be able to tell it apart from a rare rash that could be a sign of a severe reaction. Therefore, seek immediate medical attention if you develop any rash. A very serious allergic reaction to this drug is unlikely, but seek immediate medical attention if it occurs. Symptoms of a serious allergic reaction may include: rash, blisters, peeling skin, fever, 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.
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