See also Warning section.Stomach upset, nausea, vomiting, constipation, tiredness, loss of appetite/weight loss, dry mouth, dizziness, drowsiness, trouble sleeping, or decrease in sexual ability/desire may occur. In women, menstrual cramps or missed/irregular periods may also occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly.To lessen the chance of dizziness, get up slowly 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.Tell your doctor right away if you have any serious side effects, including: difficulty urinating, unusually fast/irregular heartbeat, fainting, numbness/tingling.Atomoxetine 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, stomach/abdominal pain, yellowing eyes/skin.This medication may rarely cause serious problems such as a heart attack or stroke. Get medical help right away if you experience any of the following: chest/jaw/left arm pain, shortness of breath, unusual sweating, weakness on one side of the body, confusion, slurred speech, sudden vision changes.Rarely, males (including young boys and teens) may have a painful or prolonged erection lasting 4 or more hours while using this medication. Caregivers/parents should also be watchful for this serious side effect in boys. If a painful or prolonged erection occurs, stop using this drug and get medical help right away, or permanent problems could occur. Ask your doctor or pharmacist for more details.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.