Children with ADHD who are treated with medication may experience certain side effects. The most common side effects of methylphenidate treatment are: decreased appetite, weight loss, sleep problems, headache, stomach pain, and irritability. The most common possible side effects of tomoxetine are: nausea, loss of appetite and weight loss. A small number of children may feel sleepy or irritable during the day after taking the drug. Overall, side effects are usually mild and non-threatening, and can subside with continued use of the medication or can be relieved with adjustments in treatment by the doctor.