What are the guidelines for using phentermine (phentermine) for weight loss?

Guidelines for Using Phentermine for Weight Loss

Phentermine should only be used as a short-term (a few weeks) adjunct to diet, exercise, and behavioral modification for weight management in patients with BMI ≥30 kg/m² or BMI ≥27 kg/m² with weight-related comorbidities. 1

Indications and Patient Selection

• Phentermine is FDA-approved for short-term use in:

  • Patients with BMI ≥30 kg/m²
  • Patients with BMI ≥27 kg/m² with comorbidities (e.g., controlled hypertension, diabetes, hyperlipidemia) 1

• Best candidates for phentermine treatment:

  • Patients reporting greater baseline hunger and less dietary restraint 2
  • Patients with lower income who lack access to newer, costlier medications 3

Dosing and Administration

• Standard dosing options:

  • 15-37.5 mg once daily (standard formulation)
  • 8 mg three times daily (Lomaira) 4

• Administration timing:

  • Morning dosing is typical to avoid insomnia
  • Consider afternoon dosing for patients who eat more later in the day 5

Monitoring and Follow-up

• Initial monitoring:

  • Blood pressure and heart rate every 2 weeks during first 12 weeks
  • Efficacy and safety assessment at least monthly for first 3 months 4

• Discontinuation:

  • Gradual tapering recommended to minimize withdrawal effects:
    • Week 1: Half the usual daily dose
    • Week 2: Half dose every other day
    • Week 3: Complete discontinuation 4

Contraindications

• Absolute contraindications:

  • History of cardiovascular disease
  • Uncontrolled hypertension
  • Hyperthyroidism
  • Glaucoma
  • Anxiety disorders
  • History of drug abuse
  • Use of MAO inhibitors (within 14 days)
  • Pregnancy or breastfeeding 4

• Use with caution in:

  • Patients with mild to moderate hypertension
  • Patients with depression or taking antidepressants
  • Older adults
  • Patients at risk for glaucoma 3

Adverse Effects

• Common adverse effects:

  • Dry mouth
  • Insomnia
  • Constipation
  • Headache
  • Elevated blood pressure
  • Elevated heart rate
  • Anxiety 4

• Serious adverse effects requiring immediate medical attention:

  • Dyspnea
  • Angina pectoris
  • Syncope
  • Severe hypertension 4

Duration of Treatment

• FDA-approved duration: Short-term use only (a few weeks) 1

• Important considerations:

  • The Mexican clinical practice guidelines suggest phentermine may be used for 6 months or more in most patients 3
  • Observational data suggests prolonged use (>12 months) may not significantly increase cardiovascular events at 24 months 3
  • However, the FDA label and American Diabetes Association still recommend only short-term use due to limited long-term safety data 4, 1

Drug Interactions

• Avoid concurrent use with:

  • MAO inhibitors (risk of hypertensive crisis)
  • Serotonergic drugs
  • Other stimulants, including ADHD medications like Adderall 4

• May require dose adjustment of:

  • Insulin
  • Oral hypoglycemic medications in diabetic patients 4

Efficacy Expectations

• Modest weight loss outcomes:
  • Approximately 3.6 kg at 6 months
  • 37-46% of patients achieve ≥5% loss of baseline weight 4

Important Caveats

• Phentermine should never be prescribed as a stand-alone treatment without behavioral interventions 3
• Newer medications like GLP-1 receptor agonists (semaglutide, tirzepatide) may be preferred for long-term weight management due to greater efficacy and safety 4
• Patients must be informed that phentermine is a controlled substance with potential for dependence 1
• Combination therapy with topiramate may provide greater weight loss benefits but requires careful monitoring for additional side effects 6, 7