What are the guidelines for prescribing and managing phentermine (Adipex-P) for weight loss?

Guidelines for Prescribing and Managing Phentermine for Weight Loss

Phentermine should only be prescribed as a short-term adjunct (a few weeks) for weight reduction in patients with BMI ≥30 kg/m² without comorbidities or BMI ≥27 kg/m² with weight-related comorbidities, alongside exercise, behavioral modification, and caloric restriction. 1, 2

Patient Selection Criteria

• BMI Requirements:

  • BMI ≥30 kg/m² (without comorbidities) OR
  • BMI ≥27 kg/m² with weight-related comorbidities (e.g., hypertension, diabetes, hyperlipidemia) 1, 2

• Contraindications:

  • History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, heart failure, uncontrolled hypertension)
  • 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 2
  • Age <16 years (not recommended) 1

Dosage and Administration

• Standard dosing:

  • 15-30 mg once daily, typically taken approximately 2 hours after breakfast 1
  • Alternative: 8 mg three times daily (Lomaira) 1
  • Late evening administration should be avoided (risk of insomnia) 2

• Special populations:

  • Severe renal impairment (eGFR 15-29 mL/min/1.73m²): Maximum 15 mg daily
  • Avoid in patients with eGFR <15 mL/min/1.73m² or end-stage renal disease requiring dialysis 2

Monitoring Protocol

• Initial period (first 12 weeks):

  • Blood pressure and heart rate every 2 weeks 1
  • Efficacy and safety assessment at least monthly 1

• Expected outcomes:

  • Average weight loss: approximately 5-6% of body weight at 28 weeks
  • 37-46% of patients achieve ≥5% loss of baseline weight 1

• Adverse effects to monitor:

  • Common: Headache, elevated blood pressure, elevated heart rate, insomnia, dry mouth, constipation, anxiety, palpitations 1
  • Serious (requiring immediate attention): Dyspnea, angina pectoris, syncope, severe hypertension 1

Duration of Treatment

• Phentermine is indicated only for short-term use (a few weeks) 1, 2
• Long-term use (beyond 12 weeks) may be associated with:
  • Development of tolerance to the anorectic effect
  • Cardiovascular side effects
  • Risk of dependence and withdrawal symptoms
  • Potential for drug-drug interactions 1

Discontinuation Protocol

When discontinuing phentermine, gradual tapering is recommended:

• Week 1: 18.75 mg daily (half tablet)
• Week 2: 18.75 mg every other day
• Week 3: Discontinue completely 1

Drug Interactions

• Avoid concomitant use with:

  • Other weight loss medications (safety and efficacy not established) 2
  • Serotonergic drugs (including SSRIs) 1, 2
  • MAO inhibitors (risk of hypertensive crisis) 2
  • Other stimulants, including ADHD medications like Adderall (significant cardiovascular dangers) 1

• Dose adjustments may be needed for:

  • Insulin or oral hypoglycemic medications in diabetic patients 1, 2

Clinical Pearls and Pitfalls

• Predictors of better response: Patients reporting greater hunger and less dietary restraint at baseline may achieve more significant weight loss with phentermine 3

• Alternatives for long-term management: Consider GLP-1 receptor agonists (semaglutide, tirzepatide) for long-term weight management, especially in patients with cardiovascular disease 1

• Cardiovascular risk: The American Heart Association recommends avoiding phentermine and using semaglutide instead when cardiovascular disease is present 1

• Abuse potential: Phentermine is a Schedule IV controlled substance; prescribe the least amount feasible to minimize the possibility of overdosage 2

While some recent research suggests potential safety with longer-term use 4, the FDA-approved indication and current guidelines still recommend phentermine only as a short-term intervention, with careful monitoring of cardiovascular parameters throughout treatment.