Atomoxetine Dosing in Adults
For adults, start atomoxetine at 40 mg daily, increase after a minimum of 3 days to a target dose of 80 mg daily (given once daily or divided twice daily), with a maximum of 100 mg daily. 1
Standard Adult Dosing
- Initial dose: 40 mg/day
- Target dose: 80 mg/day (after minimum 3 days)
- Maximum dose: 100 mg/day
- Administration: Can be given once daily or divided into two doses
This dosing applies to adults and adolescents/children weighing over 70 kg 1.
Critical Dose Adjustments
CYP2D6 Poor Metabolizers (PMs)
Reduce the initial and target doses to 50% of the normal dose in patients genotyped as CYP2D6 poor metabolizers 1. This adjustment is essential because CYP2D6 PMs have approximately 10-fold higher atomoxetine exposure compared to extensive metabolizers, with plasma half-life extending from 5.2 hours to 21.6 hours 2. Real-world therapeutic drug monitoring data confirms that CYP2D6 PMs require approximately 1/10th the dose of normal metabolizers 3.
- Initial dose for PMs: 20 mg/day
- Target dose for PMs: 40 mg/day
- Maximum dose for PMs: 50 mg/day
Strong CYP2D6 Inhibitors
Apply the same 50% dose reduction when atomoxetine is co-administered with strong CYP2D6 inhibitors (such as paroxetine, fluoxetine, or quinidine) 1. Paroxetine increases atomoxetine AUC by 6.5-fold and maximum concentration by 3.5-fold, essentially converting extensive metabolizers into phenotypic poor metabolizers 4. Among genotypic normal metabolizers prescribed CYP2D6 inhibitors, emergency department visits increased significantly (odds ratio 1.49) 5, highlighting the clinical importance of this interaction.
Moderate Hepatic Impairment (Child-Pugh Class B)
Reduce the initial and target doses to 50% of normal in patients with moderate hepatic impairment 1. Atomoxetine exposure (AUC) increases 2-fold in moderate hepatic impairment due to decreased CYP2D6 activity and reduced hepatic blood flow 6.
- Initial dose: 20 mg/day
- Target dose: 40 mg/day
Severe Hepatic Impairment (Child-Pugh Class C)
Reduce the initial and target doses to 25% of normal in patients with severe hepatic impairment 1. Atomoxetine exposure increases 4-fold in severe hepatic impairment 6.
- Initial dose: 10 mg/day
- Target dose: 20 mg/day
Additional Considerations
CYP2C19 genotype provides additional refinement: Patients carrying CYP2C19*2 (reduced-function allele) have 1.5-fold higher atomoxetine exposure regardless of CYP2D6 genotype 3. When both CYP2D6 and CYP2C19 reduced-function genotypes are present, consider reducing the dose by an additional one-third beyond the CYP2D6-based adjustment.
No adjustment needed for renal impairment: Atomoxetine can be administered using normal dosing in patients with end-stage renal disease or lesser degrees of renal insufficiency 1.
Common Pitfalls
- Avoid rapid dose escalation: Increasing the dose too quickly causes initial somnolence and gastrointestinal symptoms 7
- Monitor cardiovascular parameters: Atomoxetine increases heart rate and blood pressure; obtain cardiac history before initiating therapy 7
- Watch for suicidal ideation: FDA black box warning requires monitoring for suicidality, particularly when starting therapy 1
- Screen for bipolar disorder: Prior to initiation, screen patients to avoid precipitating manic episodes 1
- Discontinue if hepatotoxicity occurs: Extremely rare hepatitis has been reported; stop atomoxetine if jaundice or laboratory evidence of liver injury develops 7, 1