Phentermine 15mg Dosing for Weight Loss
Start phentermine at 15 mg orally once daily in the morning (approximately 2 hours after breakfast), and titrate up to 37.5 mg daily if needed based on response and tolerability. 1, 2
Standard Dosing Protocol
The FDA-approved dosing range is 15 to 30 mg once daily, taken approximately 2 hours after breakfast. 2 However, current guidelines support a broader range:
- Starting dose: 15 mg once daily in the morning 1, 2
- Titration options: Can increase to 18.75 mg (half tablet), then to 37.5 mg based on efficacy and tolerability 1
- Lower starting doses: Some patients respond adequately to 8-9.375 mg daily 1
- Alternative formulation (Lomaira): Can be dosed up to 3 times daily, approximately 30 minutes before meals 1
Avoid late evening dosing due to insomnia risk. 1, 2
Duration and Efficacy Assessment
Phentermine is FDA-approved for short-term use only (up to 12 weeks), though many practitioners prescribe it off-label for longer durations. 3, 1 The drug is a Schedule IV controlled substance. 1
Discontinue phentermine if the patient achieves ≤5% weight loss at 12 weeks. 1 Expected outcomes with 15 mg daily over 28 weeks include:
- 46% of patients achieve ≥5% weight loss 1
- 20.8% achieve ≥10% weight loss 1
- Mean weight loss of 5.1% compared to placebo 1
Research supports that 30 mg is more effective than 15 mg at 3 months, but the difference disappears by 6 months. 4 Approximately 40% of 3-month non-responders may still achieve ≥5% weight loss if continued to 6 months. 4
Dose Adjustments for Renal Impairment
Maximum dose is 15 mg daily for severe renal impairment (eGFR 15-29 mL/min/1.73 m²). 2
Avoid phentermine entirely in end-stage renal disease (eGFR <15 mL/min/1.73 m²) or dialysis patients. 2
Absolute Contraindications
Phentermine is contraindicated in: 3, 1, 2
- History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, heart failure, uncontrolled hypertension)
- Current or recent (within 14 days) MAOI use
- Hyperthyroidism
- Glaucoma
- Pregnancy and breastfeeding
- History of drug abuse
- Agitated states or anxiety disorders
Common Adverse Effects
The most frequent side effects include: 3, 1
- Dry mouth
- Insomnia
- Elevated blood pressure and heart rate
- Headache
- Constipation
- Anxiety and restlessness
- Palpitations
Monitor for cardiovascular effects including tachycardia, elevated blood pressure, and potential ischemic events. 3 Long-term use may lead to pharmacological tolerance, dependence, and withdrawal symptoms. 3
Clinical Pearls
Ideal candidates are younger patients without cardiovascular disease, no uncontrolled hypertension, and those needing appetite suppression assistance. 1 Avoid in patients with baseline anxiety or insomnia as these may be exacerbated. 1
For patients with cardiovascular concerns, safer alternatives include GLP-1 receptor agonists (semaglutide) or orlistat. 1, 5 Phentermine should not be combined with other weight loss medications, including SSRIs, due to lack of safety data. 2