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

Guidelines for Using Phentermine (Adipex-P) for Weight Loss

Phentermine should be used as a short-term adjunct (12 weeks) in a weight reduction regimen based on exercise, behavioral modification, and caloric restriction for patients with BMI ≥30 kg/m² or ≥27 kg/m² with weight-related complications. 1

Patient Selection and Indications

• Appropriate candidates:
  • BMI ≥30 kg/m² or
  • BMI ≥27 kg/m² with weight-related complications (e.g., hypertension, diabetes, hyperlipidemia) 2, 1
• Must be used in conjunction with lifestyle modifications (diet and exercise)
• FDA-approved for short-term use only (12 weeks) 2, 1

Dosing and Administration

• Recommended dosage: 15-37.5 mg orally once daily 2
• Dosage should be individualized to achieve adequate response with lowest effective dose:
  • Some patients may respond to 9.375 mg (quarter tablet) or 18.75 mg (half tablet)
  • Alternative dosing: half tablet (18.75 mg) twice daily 2
• Best taken in the morning to avoid insomnia

Contraindications

Phentermine is absolutely contraindicated in patients with:

• History of cardiovascular disease 2, 3
• Hyperthyroidism 2, 1
• Glaucoma 2, 1
• Agitated states 2, 1
• History of drug abuse 2, 1
• During or within 14 days of taking MAOIs 2, 1
• Pregnancy or nursing 2, 1
• Concomitant use of other sympathomimetic amines 2, 3, 1

Monitoring Requirements

• Blood pressure and heart rate should be monitored periodically 2, 3
  • Recommended frequency: at least every 2 weeks during the first 12 weeks 3
• Assess efficacy and safety at least monthly for the first 3 months 2
• If less than 5% weight loss is achieved at 12 weeks, discontinue the medication 2, 1

Side Effects

Common side effects include:

• Dizziness
• Dry mouth
• Difficulty sleeping
• Irritability
• Nausea/vomiting
• Diarrhea
• Constipation 2

Important Considerations for Long-Term Use

• While FDA-approved for only 12 weeks, many practitioners use phentermine longer in an off-label fashion given the chronic nature of weight management 2
• Limited long-term safety data exists on cardiovascular outcomes with extended use 3
• When discontinuing phentermine after extended use, tapering is recommended:
  • Week 1: Reduce to half the dose
  • Week 2: Further reduce to every other day
  • Week 3: Discontinue completely 3

Drug Interactions

• Avoid concurrent use with:
  • Other weight loss medications 1
  • Serotonergic drugs (risk of serotonin syndrome) 3
  • Tramadol (severe potential drug-drug interaction) 4
  • Alcohol 1
• May require reduction in dose of insulin or oral hypoglycemic medications in diabetic patients 1

Special Populations

Adolescents

• Phentermine monotherapy is not FDA-approved for patients 16 years and under 3, 5
• Limited data suggests it may be effective in adolescents when combined with lifestyle interventions 6
• Consider FDA-approved alternatives for adolescents such as orlistat or phentermine/topiramate extended-release 3

Efficacy Expectations

• Mean weight loss attributable to phentermine is modest (approximately 3-5 kg more than placebo) 2
• Expected weight loss: approximately 5.1% at 28 weeks 2

Discontinuation Criteria

• Discontinue if:
  • Less than 5% weight loss at 12 weeks 2
  • Development of new, unexplained symptoms of dyspnea, angina, syncope, or lower extremity edema 1
  • Tolerance to the anorectic effect develops 1

Phentermine should be viewed as one component of a comprehensive weight management strategy that includes dietary changes, increased physical activity, and behavioral modifications. The medication's limited usefulness should be measured against possible risks inherent in its use.