See also Warning section. Stomach upset, nausea, vomiting, constipation, fatigue, loss of appetite, dry mouth, dizziness, trouble sleeping, menstrual cycle changes, or mood changes may 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. Tell your doctor immediately if any of these unlikely but serious side effects occur: weight loss, decrease in sexual ability/desire (e.g., impotence, ejaculation problems), difficulty urinating, unusually fast/irregular heartbeat, fainting, numbness/tingling. This medication may rarely cause serious problems such as heart attacks or stroke. Seek immediate medical attention if you experience any of the following: chest/jaw/left arm pain, sudden severe headache, weakness on one side of the body, confusion, slurred speech, sudden vision changes (e.g., double vision, loss of vision). Atomoxetine has very rarely caused very serious liver problems. If you notice any of the following highly unlikely but very serious side effects, seek immediate medical attention: yellowing of skin or eyes, dark urine, severe stomach pain, "flu-like" symptoms (e.g., fever, chills, unexplained fatigue). If you experience any of these symptoms ask your doctor about whether or not you should ever take atomoxetine again. Rarely, males may have a painful or prolonged erection lasting 4 or more hours. If this occurs, seek immediate medical attention, or permanent problems could occur. A very serious allergic reaction to this drug is rare. However, seek immediate medical attention 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.