What are the guidelines for using Phentermine (Phentermine) for weight loss in patients with obesity?

Guidelines for Using Phentermine for Weight Loss in Patients with Obesity

Phentermine is indicated only as a short-term (a few weeks) adjunct to lifestyle modifications for weight management in patients with BMI ≥30 kg/m² or BMI ≥27 kg/m² with weight-related comorbidities. 1

Patient Selection Criteria

• BMI Requirements:

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

• Absolute Contraindications:

  • History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, congestive heart failure)
  • Uncontrolled hypertension
  • During or within 14 days of monoamine oxidase inhibitor use
  • Hyperthyroidism
  • Glaucoma
  • Agitated states
  • History of drug abuse
  • Pregnancy or nursing
  • Known hypersensitivity to sympathomimetic amines 1

Dosing and Administration

• Standard Dosing:

  • 15-30 mg once daily, taken approximately 2 hours after breakfast 1
  • Alternative dosing: 8 mg three times daily (Lomaira formulation) 2
  • Avoid late evening dosing due to insomnia risk 1

• Special Populations:

  • Severe renal impairment (eGFR 15-29 mL/min/1.73m²): Maximum 15 mg daily
  • Avoid use in eGFR <15 mL/min/1.73m² or end-stage renal disease requiring dialysis 1
  • Not recommended for patients under 16 years of age 2

Monitoring and Follow-up

• Initial Period (First 12 weeks):

  • Monitor blood pressure and heart rate every 2 weeks 2
  • Assess efficacy and safety at least monthly 2

• Efficacy Assessment:

  • Target weight loss: 5% of baseline weight
  • Average weight loss with phentermine: 3.6-5.1% at 6 months 2
  • Consider discontinuation if <5% weight loss after 12 weeks of treatment 3

Safety Considerations

• Cardiovascular Monitoring:

  • Regular monitoring of blood pressure and heart rate is essential
  • Caution in patients with cardiovascular risk factors 3
  • No large cardiovascular outcome trial data exists for long-term use 3

• Common Adverse Effects:

  • Dry mouth (most common)
  • Insomnia
  • Constipation
  • Palpitations
  • Irritability
  • Headache
  • Elevated blood pressure/heart rate 2

• Serious Adverse Effects (requiring immediate attention):

  • Dyspnea
  • Angina pectoris
  • Syncope
  • Severe hypertension 2

Duration of Treatment

• FDA-Approved Duration:

  • Short-term use (a few weeks) 1
  • The AGA acknowledges that many clinicians use phentermine for longer than 3 months in off-label fashion due to the chronic nature of obesity 3

• Off-Label Extended Use:

  • If extended use is deemed necessary, document specific benefits, tolerance, and adverse effects
  • Advise patients regarding off-label use and limited supporting data 3
  • Consider tapering when discontinuing:
    • Week 1: Half dose daily
    • Week 2: Half dose every other day
    • Week 3: Complete discontinuation 2

Drug Interactions

• Avoid Concurrent Use With:

  • Monoamine oxidase inhibitors (risk of hypertensive crisis)
  • Other weight loss medications (safety not established)
  • Serotonergic agents (SSRIs)
  • Other stimulants, including ADHD medications like Adderall 2

• Potential Adjustments:

  • May require dose adjustment of insulin or oral hypoglycemics in diabetic patients 2

Alternatives for Long-Term Management

• For long-term weight management, especially in patients with cardiovascular disease, consider GLP-1 receptor agonists (semaglutide, tirzepatide) which have greater efficacy and safety data 2

Important Caveats

• Weight regain commonly occurs after medication discontinuation 2
• Phentermine is a Schedule IV controlled substance due to concerns for abuse and dependence 3
• Phentermine should not be confused with the "fen-phen" combination that caused valvular heart disease in the 1990s; phentermine alone has not been associated with these cardiac valvulopathies 3

Remember that lifestyle modifications with diet and exercise remain the foundation of any weight management program, with phentermine serving only as a short-term adjunctive therapy.