What is the recommended dose of phentermine (anorexiant) for weight loss?

Phentermine Dosing for Weight Loss

Recommended Dosage

The standard recommended dose of phentermine is 15 to 37.5 mg orally once daily in the morning, approximately 2 hours after breakfast, with the goal of using the lowest effective dose that achieves adequate appetite suppression. 1

Dosing Algorithm

Initial Dosing Strategy

• Start with 15 mg once daily in the morning for most patients to assess tolerability 2, 1
• Alternatively, begin with even lower doses (8-9.375 mg daily) for patients with cardiovascular concerns or sensitivity to stimulants 2
• Avoid late evening administration due to insomnia risk 1

Dose Titration

• If 15 mg is well-tolerated but weight loss is inadequate, increase to 30-37.5 mg once daily 3, 1
• Alternative dosing schedules include:
  • Split dosing: 18.75 mg twice daily 2
  • Low-dose formulation (Lomaira 8 mg): up to 3 times daily, approximately 30 minutes before meals 4, 2
  • Quarter tablet (9.375 mg) or half tablet (18.75 mg) once daily as intermediate options 2

Special Population Adjustments

• Severe renal impairment (eGFR 15-29 mL/min/1.73 m²): Maximum dose 15 mg daily 1
• End-stage renal disease or eGFR <15 mL/min/1.73 m²: Avoid phentermine entirely 1

Treatment Duration and Efficacy Assessment

FDA-Approved Duration

• Phentermine is FDA-approved only for short-term use (up to 12 weeks) as a Schedule IV controlled substance 4, 2, 1
• Many clinicians prescribe it off-label for longer durations (3-6 months or more) for continued weight management 4, 2

Discontinuation Criteria

• Discontinue if less than 5% weight loss is achieved after 12 weeks on maximum tolerated dose 4, 2
• Expected outcomes: At 28 weeks with 15 mg daily, 46% of patients achieve ≥5% weight loss and 21% achieve ≥10% weight loss 2
• Mean weight loss is approximately 3.6 kg over 2-24 weeks compared to placebo 3

Long-Term Considerations

• Research shows that 30 mg is more effective than 15 mg at 3 months, but efficacy equalizes by 6 months 5
• Approximately 40% of 3-month non-responders may achieve ≥5% weight loss if treatment continues to 6 months 5
• Tolerance typically develops within a few weeks; if this occurs, discontinue rather than exceeding recommended doses 1

Pre-Treatment Requirements

Mandatory Screening

• Cardiovascular assessment: Measure baseline blood pressure and heart rate, as phentermine causes mild sympathetic activation 4
• Pregnancy testing: Obtain baseline β-HCG in women of reproductive potential, as phentermine is contraindicated in pregnancy 4, 1
• Renal function: Check eGFR to guide dosing adjustments 1
• Thyroid status: Exclude hyperthyroidism 4
• Ophthalmologic history: Screen for glaucoma 4
• Psychiatric assessment: Evaluate for anxiety disorders or agitated states 4
• Medication review: Ensure no MAOI use within 14 days 4, 1

Absolute Contraindications

Phentermine must be avoided in patients with: 3, 4, 1

• History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, heart failure, uncontrolled hypertension)
• Current or recent (within 14 days) MAOI use
• Hyperthyroidism
• Glaucoma
• Pregnancy or breastfeeding
• History of drug abuse
• Agitated states or severe anxiety disorders

Common Adverse Effects and Monitoring

Expected Side Effects

• Dry mouth, insomnia, dizziness, irritability, constipation, and palpitations are most common 2
• Cardiovascular effects include elevated blood pressure, elevated heart rate, tachycardia, and rarely ischemic events 3
• Central nervous system effects include overstimulation, restlessness, euphoria, dysphoria, tremor, headache, and rarely psychosis 3

Monitoring Schedule

• Baseline: Blood pressure, heart rate, weight, BMI 4
• Periodic monitoring: Blood pressure and heart rate at follow-up visits 4
• Weight assessment: Evaluate response at 12 weeks to determine continuation 4, 2

Clinical Pearls

Optimal Patient Selection

• Ideal candidates: Younger patients without active coronary disease, controlled blood pressure, and need for appetite suppression assistance 2
• Avoid in: Patients with cardiovascular disease (consider orlistat instead), anxiety disorders, or insomnia 2

Practical Considerations

• Phentermine can be taken with or without food 1
• For patients who skip breakfast and eat more in afternoon/evening, consider afternoon dosing to target high-risk eating periods 6
• Concomitant alcohol use may result in adverse drug reactions and should be avoided 1
• Insulin or oral hypoglycemic requirements may decrease with weight loss; monitor glucose closely in diabetic patients 1
• Prescribe the least amount feasible at one time to minimize overdosage risk given abuse potential 1