What should be done for an adult or adolescent patient with Attention Deficit Hyperactivity Disorder (ADHD) who experiences dizziness, vomiting, and lightheadedness after their second dose of atomoxetine (18mg)?

Management of Acute Adverse Effects After Second Dose of Atomoxetine

Discontinue atomoxetine immediately and do not restart this medication. The patient is experiencing common but intolerable adverse effects (dizziness, vomiting, lightheadedness) that warrant immediate cessation, particularly given this occurred after only the second dose at a low starting dose of 18mg 1, 2.

Immediate Actions

Discontinue Atomoxetine

• Stop atomoxetine immediately without tapering, as the patient has only taken two doses and tapering is not necessary with such brief exposure 3, 4.
• The FDA label confirms that dizziness is a common adverse effect in adults, and vomiting is among the most common adverse effects in both pediatric and adult populations 2.
• These symptoms (dizziness, vomiting, lightheadedness) are consistent with the known adverse effect profile, particularly gastrointestinal symptoms and somnolence that occur when dosage is increased too rapidly 1.

Assess for Serious Complications

• Evaluate for orthostatic hypotension immediately by measuring blood pressure and heart rate in both supine and standing positions, as orthostatic hypotension and syncope have been reported with atomoxetine 2.
• The FDA warns that atomoxetine should be used with caution in any condition that may predispose patients to hypotension or conditions associated with abrupt heart rate or blood pressure changes 2.
• Document baseline vital signs for comparison if future ADHD medications are considered 1.

Consider CYP2D6 Poor Metabolizer Status

• This patient may be a CYP2D6 poor metabolizer (PM), which occurs in approximately 7% of Caucasians and results in 10-fold higher drug exposure and significantly increased adverse effects 5, 1.
• Poor metabolizers experience higher rates of adverse effects including dizziness, nausea, vomiting, and fatigue due to dramatically elevated plasma levels 1.
• If atomoxetine is ever reconsidered in the future, CYP2D6 genotyping should be performed and dose adjustment would be mandatory 2.

Symptomatic Management

• Provide supportive care for nausea and vomiting with antiemetics if needed 6.
• Ensure adequate hydration, particularly if vomiting has been significant 6.
• Monitor for resolution of symptoms, which should occur within 24-48 hours as the drug is eliminated 1.
• Advise the patient to avoid sudden position changes until lightheadedness resolves to prevent falls 2.

Next Steps for ADHD Treatment

Alternative Medication Selection

• Consider switching to a stimulant medication (methylphenidate or amphetamine derivatives) as first-line therapy, as stimulants have larger effect sizes and are recommended as first-line by the American Academy of Pediatrics 1.
• Atomoxetine is positioned as second-line therapy due to smaller effect size and delayed onset of action (6-12 weeks) 1.
• If stimulants are contraindicated or refused, consider extended-release guanfacine or clonidine as alternative non-stimulant options 1.

Timing of New Medication

• Allow at least 1 week washout period before initiating another ADHD medication to ensure complete elimination of atomoxetine 3.
• This washout period is particularly important given the possibility of poor metabolizer status with prolonged half-life 1.

Critical Pitfalls to Avoid

• Do not attempt to "push through" these side effects or reduce the dose further - the patient has already demonstrated intolerance at a very low starting dose of 18mg, which is below the typical starting dose of 40mg for adults over 70kg 1, 2.
• Do not restart atomoxetine in this patient - early intolerance at low doses predicts poor tolerability and the risk-benefit ratio does not favor continuation 1, 3.
• Do not delay switching to an alternative medication - stimulants are more effective and better tolerated in most patients, and this patient has already lost time with an ineffective trial 1.
• Do not ignore the cardiovascular monitoring - orthostatic vital signs are essential as syncope has been reported in 0.8% of atomoxetine-treated patients 2.