What is the proper way to initiate phentermine (phentermine) therapy?

Initiating Phentermine Therapy for Weight Management

Phentermine should be started at 15 mg orally once daily in the morning, with individualized dosing adjustments based on response and tolerability, while avoiding late evening administration to prevent insomnia. 1

Patient Selection and Dosing

• Appropriate candidates:

  • BMI ≥30 kg/m² or BMI ≥27 kg/m² with weight-related comorbidities (hypertension, diabetes, hyperlipidemia) 2, 1
  • Patients needing assistance with appetite suppression 2
  • Younger patients without cardiovascular disease 2

• Contraindications: 1

  • History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, heart failure, uncontrolled hypertension)
  • Use of MAOIs or within 14 days of MAOI use
  • Hyperthyroidism, glaucoma, agitated states
  • History of drug abuse
  • Pregnancy or nursing
  • Known hypersensitivity to sympathomimetic amines

• Dosing protocol:

  • Initial dose: 15 mg once daily in the morning (approximately 2 hours after breakfast) 2, 1
  • Dosage range: 15-37.5 mg daily 2
  • For some patients, 9.375 mg (quarter tablet) or 18.75 mg (half tablet) may be adequate 2
  • Alternative: Low-dose 8 mg tablet can be prescribed up to 3 times per day 2
  • For severe renal impairment (eGFR 15-29 mL/min/1.73 m²): Limit to 15 mg daily 1
  • Avoid use in patients with eGFR <15 mL/min/1.73 m² 1

Administration Considerations

• Take early in the morning to minimize insomnia 3, 1
• Can be taken with or without food 1
• Avoid late evening administration due to risk of insomnia 2, 1
• FDA-approved for short-term use only (up to 12 weeks) 2, 3, 1
• Some clinicians prescribe for longer durations off-label 2

Monitoring and Follow-up

• Monitor blood pressure and heart rate at each visit 3
• Particular vigilance during first 12 weeks of treatment 3
• Recommended monitoring frequency: at least every 2 weeks 3
• Watch for common side effects: 2
  • Dry mouth
  • Difficulty sleeping
  • Dizziness
  • Irritability
  • Nausea/vomiting
  • Constipation

Important Precautions

• Do not use with other weight loss medications (safety and efficacy of combinations not established) 1
• Avoid use with monoamine oxidase inhibitors due to risk of hypertensive crisis 1
• Use caution with other serotonergic drugs (risk of serotonin syndrome) 1
• May affect insulin and oral hypoglycemic medication requirements 1
• Avoid alcohol use during treatment 3, 1
• May impair ability to operate machinery or drive 1

Efficacy Expectations

• In clinical trials, phentermine monotherapy (15 mg daily) was associated with 6.0 kg weight loss at 28 weeks 2
• Approximately 46% of patients achieve ≥5% weight loss and 20.8% achieve ≥10% weight loss 2
• The goal is not only weight reduction but improvement in obesity-related comorbidities 2

Common Pitfalls to Avoid

• Starting with too high a dose (increases side effect risk)
• Administering late in the day (increases insomnia risk)
• Failing to monitor cardiovascular parameters regularly
• Using in patients with cardiovascular contraindications
• Prescribing without concurrent lifestyle modifications (diet, exercise)
• Continuing beyond FDA-approved duration without careful risk-benefit assessment

Phentermine should be used as part of a comprehensive weight management program that includes dietary changes, physical activity, and behavioral modifications for optimal outcomes.