What is the recommended dosing for phentermine (Adipex-P)?

Phentermine Dosing

The recommended dosing for phentermine is 15 to 37.5 mg orally once daily in the morning, approximately 2 hours after breakfast, with the option to start at lower doses (as low as 8-9.375 mg) and titrate based on efficacy and tolerability. 1, 2

Standard Dosing Regimens

Primary Dosing Options

• Standard dose: 15 to 37.5 mg once daily in the morning 1, 2
• Timing: Approximately 2 hours after breakfast for optimal appetite control 2
• Avoid late evening dosing due to risk of insomnia 1, 2

Flexible Dosing Strategies

Start low and titrate up to achieve adequate response with the lowest effective dose 1:

• Quarter tablet: 9.375 mg daily may be adequate for some patients 1
• Half tablet: 18.75 mg daily as an intermediate option 1
• Split dosing: Half tablet (18.75 mg) twice daily is also an option 1

Low-Dose Formulation (Lomaira)

• 8 mg scored tablets can be prescribed up to 3 times per day (approximately 30 minutes before meals) 1
• Dosing can be as low as 4 mg using the scored tablets 1
• Some clinicians use these low doses on an "as needed basis" before situations with high risk of hedonic food consumption 1

Special Populations

Renal Impairment

Maximum dose of 15 mg daily for patients with severe renal impairment (eGFR 15-29 mL/min/1.73 m²) 2

Avoid use entirely in patients with eGFR <15 mL/min/1.73 m² or end-stage renal disease requiring dialysis 2

Pediatric Patients

Not recommended for use in patients 16 years of age and under 2

Duration of Treatment

FDA-Approved Duration

• Approved for short-term use only (3 months) due to lack of long-term safety trials 1
• Phentermine is a Schedule IV controlled substance 1

Off-Label Extended Use

Many practitioners prescribe phentermine for longer durations as off-label therapy for continued weight management 1

When extending treatment beyond 3 months 1:

• Document specific benefits, tolerance, and adverse effects
• Advise patients regarding off-label use and limited data
• Confirm with state licensure authorities regarding local laws and regulations
• Assess efficacy and safety at least monthly for first 3 months, then at least every 3 months 1

Efficacy Assessment and Discontinuation

Discontinue if inadequate weight loss is achieved 1:

• If approximately 5% weight loss or less at 12 weeks, discontinue and consider alternative medication or other treatments 1

Key Contraindications

Absolute contraindications include 1, 2:

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

Patient Selection Considerations

Ideal candidates 1:

• Younger patients without active or unstable coronary disease
• No uncontrolled hypertension
• Patients needing appetite suppression assistance
• Avoid in patients with anxiety or insomnia (may be exacerbated)

Avoid in patients with cardiovascular disease - safer alternatives include lorcaserin and orlistat 1

Common Adverse Effects

Most frequent side effects include 1:

• Dry mouth
• Difficulty sleeping/insomnia
• Dizziness
• Irritability
• Constipation
• Palpitations

Expected Weight Loss

Mean weight loss: 5.1% at 28 weeks compared with placebo 1

In a 28-week trial with 15 mg daily 1:

• 46% achieved ≥5% weight loss
• 20.8% achieved ≥10% weight loss