Starting Dose for Phentermine
The recommended starting dose for phentermine is 15 mg to 30 mg taken once daily, approximately 2 hours after breakfast for appetite control. 1
Dosage Guidelines
- The FDA-approved dosing for phentermine should be individualized to obtain adequate response with the lowest effective dose:
- Standard starting dose: 15-30 mg once daily in the morning 1
- Administration timing: Approximately 2 hours after breakfast 1
- Alternative dosing: One 30 mg capsule daily has been found adequate for appetite suppression for 12-14 hours 1
- For patients with severe renal impairment (eGFR 15-29 mL/min/1.73m²): Maximum 15 mg daily 1
Important Administration Considerations
- Late evening medication should be avoided due to the risk of insomnia 1, 2
- Phentermine is not recommended for patients under 16 years of age 1
- Phentermine is indicated as a short-term (a few weeks) adjunct to diet and lifestyle modification 1, 2
Patient Selection Criteria
Phentermine is indicated for patients with:
- BMI ≥30 kg/m² without comorbidities, OR
- BMI ≥27 kg/m² with weight-related comorbidities (e.g., hypertension, diabetes, hyperlipidemia) 2
Monitoring and Safety
- Blood pressure and heart rate should be monitored every 2 weeks during the first 12 weeks 2
- Efficacy and safety assessment should be conducted at least monthly for the first 3 months 2
- Common adverse effects include headache, elevated blood pressure, elevated heart rate, insomnia, dry mouth, constipation, and anxiety 3, 2
Contraindications
Phentermine is contraindicated in patients with:
- History of cardiovascular disease
- During or within 14 days of MAO inhibitor use
- Hyperthyroidism
- Glaucoma
- Agitated states
- History of drug abuse
- Pregnancy or nursing
- Known hypersensitivity to sympathomimetic amines 1
Clinical Pearls
- Weight regain commonly occurs after medication discontinuation 2
- When discontinuing phentermine, gradual tapering is recommended to minimize withdrawal effects 2
- Phentermine produces modest weight loss (approximately 3.6 kg at 6 months) 2
- Some clinicians have found that starting at a lower dose (such as 15 mg) and titrating up based on response and tolerability may help minimize side effects 4
Caution
Avoid use in patients with:
- eGFR less than 15 mL/min/1.73m² or end-stage renal disease requiring dialysis 1
- History of seizure disorders as phentermine may lower seizure threshold 2
- Concurrent use of serotonergic drugs or other stimulants 2
By following these dosing guidelines and monitoring parameters, phentermine can be used effectively and safely as a short-term adjunct for weight management in appropriate patients.