What is the recommended dosing for Phentermine (Fentermina) for weight loss?

Phentermine Dosing for Weight Loss

The recommended dosage of phentermine for weight loss is 15 to 37.5 mg orally once daily, taken approximately 2 hours after breakfast for appetite control. 1, 2, 3

Dosing Guidelines

• Starting dose: Begin with the lowest effective dose (15 mg daily) and titrate based on response and tolerability 2, 3
• Alternative dosing options:
  • Half tablet (18.75 mg) twice daily 2
  • Low-dose 8 mg tablet up to 3 times per day (brand name Lomaira) 2
  • For patients with severe renal impairment (eGFR 15-29 mL/min/1.73m²): Maximum 15 mg daily 1

Administration Timing

• Preferred timing: Morning or approximately 2 hours after breakfast 1, 3
• Avoid late evening dosing: To prevent insomnia 1, 2
• Duration of effect: A single 30 mg dose provides appetite suppression for 12-14 hours 1

Patient Selection and Contraindications

Phentermine is indicated for patients with:

• BMI ≥30 kg/m² or
• BMI ≥27 kg/m² with weight-related comorbidities (hypertension, diabetes, hyperlipidemia) 3, 1

Absolute contraindications:

• Cardiovascular disease (coronary artery disease, stroke, arrhythmias, heart failure, uncontrolled hypertension)
• During or within 14 days of MAOI use
• Hyperthyroidism
• Glaucoma
• Agitated states
• History of drug abuse
• Pregnancy or nursing
• Known hypersensitivity to sympathomimetic amines 1, 3, 2

Monitoring and Efficacy Assessment

• Assess efficacy and safety at least monthly for the first 3 months 2, 3
• Monitor blood pressure and heart rate every 2 weeks during the first 12 weeks 3
• Discontinuation criteria: If less than 5% weight loss is achieved at 12 weeks 2, 3

Expected Outcomes

• Mean weight loss: Approximately 5-6% of body weight at 28 weeks 2
• In clinical trials, phentermine alone (15 mg daily) was associated with a 6.0 kg weight loss at 28 weeks compared to placebo 2
• About 46% of patients achieve ≥5% weight loss and 20.8% achieve ≥10% weight loss 2

Common Side Effects

• Dry mouth
• Difficulty sleeping
• Dizziness
• Irritability
• Nausea/vomiting
• Diarrhea
• Constipation 2, 3

Important Considerations

• Phentermine is FDA-approved only for short-term use (up to 3 months), although many practitioners prescribe it for longer durations in clinical practice 2
• It is a Schedule IV controlled substance 2
• Not recommended for patients under 16 years of age 1
• May require dose reduction of insulin or oral hypoglycemic medications in diabetic patients 3

Tapering Recommendations

If discontinuing phentermine, consider gradual tapering to minimize withdrawal effects:

1. Week 1: Reduce to half the current dose
2. Week 2: Further reduce to every other day dosing
3. Week 3: Discontinue completely 3

Phentermine remains the most commonly prescribed anti-obesity medication in the United States, but its use should be carefully considered as part of a comprehensive weight management program that includes diet, exercise, and behavioral modification 2, 1.