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

Guidelines for Using Phentermine for Weight Loss

Phentermine should be used as a short-term (12 weeks) adjunct to lifestyle modifications for adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with weight-related complications, though many practitioners use it off-label for longer periods given the chronic nature of weight management. 1, 2

Indications and Patient Selection

• Phentermine is FDA-approved as a short-term (few weeks) adjunct in weight reduction regimens based on exercise, behavioral modification, and caloric restriction 2
• Appropriate for patients with:
  • BMI ≥30 kg/m² OR
  • BMI ≥27 kg/m² with weight-related complications (e.g., hypertension, diabetes, hyperlipidemia) 2, 1
• The American Gastroenterological Association (AGA) suggests using phentermine with lifestyle modifications compared to lifestyle modifications alone (conditional recommendation, low-quality evidence) 1

Dosing and Administration

• Recommended dosage: 15-37.5 mg orally once daily, preferably in the morning 1
• Start with the lowest effective dose and individualize to achieve adequate response 1
• For some patients, 9.375 mg (quarter tablet) or 18.75 mg (half tablet) may be adequate 1
• A split dose of half tablet twice daily is also an option 1
• Low-dose phentermine (8 mg) can be prescribed up to 3 times per day 3

Duration of Treatment

• FDA-approved for short-term use (12 weeks) 1, 2
• However, given the chronic nature of weight management, many practitioners use phentermine longer than 12 weeks in an off-label fashion 1
• Patients must be informed that phentermine is intended as a short-term adjunct to lifestyle modifications 2

Contraindications and Precautions

• Phentermine should be avoided in patients with:
  • History of cardiovascular disease 1
  • Uncontrolled hypertension 3
  • During or within 14 days of taking MAOIs 1, 3
  • Other sympathomimetic amines 1
  • Hyperthyroidism 1
  • Glaucoma 1
  • Agitated states 1
  • History of drug abuse 1
  • Pregnancy or nursing 1, 2

Monitoring

• Blood pressure and heart rate should be monitored periodically while taking phentermine 1, 4
• More frequent monitoring is recommended in the first 12 weeks of treatment 4
• If there is less than 5% weight loss at 12 weeks on maximum dose, consider discontinuing the medication 3

Side Effects and Risks

• Common side effects include:
  • Dizziness, dry mouth, difficulty sleeping, irritability 1
  • Nausea/vomiting, diarrhea, constipation 1
  • Insomnia, anxiety 4
• Serious risks include:
  • Development of primary pulmonary hypertension 2
  • Development of serious valvular heart disease 2
  • Effects on ability to engage in potentially hazardous tasks 2
  • Increase in blood pressure 2
  • Potential for developing tolerance 2
  • Risk of dependence and potential consequences of abuse 2

Drug Interactions

• Phentermine should not be used with:
  • MAOIs (risk of hypertensive crisis) 4, 3
  • Other sympathomimetic amines 1
• Caution with concomitant use of other medications that may interact with phentermine 5
• Common potential drug-drug interactions include those with dextromethorphan and tramadol 5

Implementation Considerations

• Taking phentermine early in the day can help minimize insomnia 4
• Store phentermine in a safe place to prevent theft, accidental overdose, misuse, or abuse 2
• Selling or giving away phentermine is against the law 2
• For patients with cardiovascular concerns, orlistat may be a safer alternative 3

Special Populations

• Limited data exists on phentermine use in adolescents, though some case series suggest it may be effective and safe when combined with lifestyle interventions 6

Efficacy

• The AGA suggests phentermine can be effective for weight loss when combined with lifestyle modifications 1
• Some studies suggest that 30 mg phentermine may be more effective than 15 mg in the first 3 months, but this difference may not persist at 6 months 7