What is the recommended dose of phentermine (Adipex-P) for weight loss?

Phentermine Dosing for Weight Loss

Recommended Dose

The standard recommended dose of phentermine for weight loss is 15 to 37.5 mg orally once daily, taken approximately 2 hours after breakfast, with the option to start at lower doses (8-15 mg) and titrate upward based on response and tolerability. 1

Dosing Algorithm

Starting Dose Options

• Standard approach: Begin with 15 mg once daily in the morning (approximately 2 hours after breakfast) 1
• Lower starting doses for patients sensitive to stimulants or with cardiovascular concerns:
  • 8 mg (using low-dose formulation) 2
  • 9.375 mg (quarter tablet) 2
• Maximum dose: 37.5 mg once daily 3, 1

Titration Strategy

• Start with the lowest effective dose and titrate upward to achieve adequate appetite suppression with minimal side effects 2, 1
• Intermediate dosing options include:
  • 18.75 mg once daily (half tablet) 2
  • Split dosing: 18.75 mg twice daily 2
• The low-dose formulation (Lomaira, 8 mg) can be dosed up to 3 times daily, approximately 30 minutes before meals 2

Timing Considerations

• Avoid late evening dosing due to risk of insomnia 1
• Administration 2 hours after breakfast provides 12-14 hours of appetite control 1
• For patients who skip breakfast and eat more in afternoon/evening, consider afternoon dosing to target high-risk eating periods 4

Duration of Treatment

• FDA-approved duration: Short-term use only (up to 12 weeks/3 months) 3, 2, 1
• Off-label practice: Many practitioners prescribe phentermine for longer durations (beyond 3 months) for continued weight management, though this lacks long-term safety data 2, 5
• Phentermine is a Schedule IV controlled substance 2

Efficacy Assessment and Discontinuation Criteria

Discontinue phentermine if the patient achieves ≤5% weight loss at 12 weeks on maximum tolerated dose. 2, 5

Expected Weight Loss

• Mean weight loss: 5.1-5.45% at 28 weeks compared to placebo 3, 2
• At 28 weeks with 15 mg daily: 46% achieve ≥5% weight loss and 20.8% achieve ≥10% weight loss 2
• Mean absolute weight loss: 3.6 kg over 2-24 weeks 3

Special Populations

Renal Impairment

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

Age Restrictions

• Not recommended for patients under 16 years of age 1

Absolute Contraindications

• History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled hypertension) 1
• During or within 14 days of MAOI use 3, 1
• Hyperthyroidism 3, 1
• Glaucoma 3, 1
• Pregnancy and breastfeeding 3, 1
• History of drug abuse 3, 1
• Agitated states or anxiety disorders 3, 1

Ideal Candidates vs. Poor Candidates

Good Candidates

• Younger patients without active or unstable coronary disease 3, 2
• Patients needing assistance with appetite suppression 3
• Patients with higher baseline hunger ratings and lower cognitive restraint (these predict better response) 6
• Controlled hypertension (not uncontrolled) 3

Poor Candidates

• Patients with uncontrolled hypertension or active coronary disease 3, 2
• Patients with anxiety, insomnia, or sensitivity to stimulants 3, 2
• Patients with history of drug abuse 3
• Recent MAOI use 3

Common Adverse Effects

• Dry mouth 3, 2
• Insomnia/difficulty sleeping 3, 2
• Dizziness 3, 2
• Irritability 2
• Constipation 3, 2
• Palpitations and elevated heart rate 3, 2
• Elevated blood pressure 3
• Headache 3

Monitoring Requirements

• Baseline blood pressure and heart rate 5
• Periodic monitoring of blood pressure and heart rate during treatment 5
• Weight assessment at 12 weeks to determine continuation 2, 5
• Screen for cardiovascular disease before initiation 5
• Assess pregnancy status in women of reproductive potential 5
• Evaluate renal function before prescribing 5

Clinical Pearls

A common pitfall is starting at maximum dose (37.5 mg) in all patients—this increases side effects without necessarily improving efficacy. Starting at 15 mg or lower and titrating allows for better tolerability while achieving similar weight loss outcomes at 6 months. 7

Another key consideration: While 30 mg phentermine may be more effective than 15 mg at 3 months, this difference disappears by 6 months of treatment, suggesting that lower doses may be equally effective with longer duration. 7