Guidelines for Prescribing Phentermine for Weight Loss
Phentermine should be prescribed as a short-term adjunct (typically a few weeks) in a comprehensive weight reduction regimen that includes exercise, behavioral modification, and caloric restriction for patients with BMI ≥30 kg/m² or BMI ≥27 kg/m² with weight-related comorbidities. 1
Patient Selection Criteria
BMI requirements:
Contraindications (do NOT prescribe if patient has):
- History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, heart failure, uncontrolled hypertension) 1
- During or within 14 days of taking monoamine oxidase inhibitors 2, 1
- Hyperthyroidism 2, 1
- Glaucoma 1
- Agitated states 1
- History of drug abuse 1
- Pregnancy or nursing 1
- Known hypersensitivity to sympathomimetic amines 1
Dosing Guidelines
- Standard dosing: 15-37.5 mg orally once daily, preferably in the morning to minimize insomnia risk 2
- Low-dose option: 8 mg tablets can be dosed up to 3 times daily (approximately 30 minutes before meals) 2
- Individualized dosing: Start with lowest effective dose and titrate as needed 1
- Renal impairment: Limit to 15 mg daily for patients with severe renal impairment (eGFR 15-29 mL/min/1.73 m²) 1
- Avoid in patients with eGFR <15 mL/min/1.73 m² or end-stage renal disease requiring dialysis 1
Duration of Treatment
- FDA approval: Short-term use (few weeks) 1
- Clinical practice: Many providers prescribe phentermine for longer durations as off-label therapy 2
- Recent evidence: Observational data suggests that prolonged use (>12 months) does not significantly increase the likelihood of major cardiovascular events at 24 months 2
Monitoring and Follow-up
- Initial follow-up: Assess efficacy and safety at least monthly for the first 3 months 2
- Ongoing monitoring: At least every 3 months thereafter 2
- Blood pressure and heart rate: Monitor periodically, especially in patients with controlled hypertension 2
- Efficacy assessment: If <5% weight loss is not achieved after 12 weeks, consider discontinuation or alternative treatment 2
Special Considerations
Cardiovascular Safety
- While historical concerns exist about cardiovascular safety, observational data from phentermine monotherapy do not show significant increases in blood pressure or heart rate in treated individuals 2
- However, caution is still advised, and phentermine should be avoided in patients with history of cardiovascular disease or uncontrolled hypertension 2
Drug Interactions
- Severe interactions to avoid:
- Moderate interactions to monitor:
- Dextromethorphan and other medications with potential interactions 3
- Insulin/oral hypoglycemics: May require dose reduction in some patients 1
Common Side Effects
- Dry mouth, insomnia, dizziness, irritability, constipation, palpitations 2
- Most adverse events are mild and resolve with dose reduction or discontinuation 4
Practical Prescribing Tips
- Prescribe early in the day to minimize insomnia risk 2
- Always prescribe as part of a comprehensive weight management program including diet, exercise, and behavioral modification 2
- Consider lower doses in patients with anxiety or insomnia as phentermine may exacerbate these conditions 2
- Despite being a Schedule IV controlled substance, recent research suggests low risk of abuse or psychological dependence even with long-term use 5
- Be aware of rare but serious potential adverse effects including psychosis, which has been reported in case studies 6
Patient Education Points
- Emphasize that phentermine is an adjunct to, not a replacement for, lifestyle changes
- Inform patients about potential side effects and when to contact their healthcare provider
- Advise patients that phentermine may impair their ability to operate machinery or drive 1
- Caution against concomitant alcohol use, which may result in adverse drug reactions 1
By following these guidelines, phentermine can be safely and effectively prescribed as part of a comprehensive weight management program for appropriate patients.