What is the recommended dosage and duration of phentermine (phentermine) treatment?

Phentermine Dosing Frequency and Duration

Phentermine is FDA-approved only for short-term use of up to 3 months (12 weeks), administered once daily at 15-37.5 mg in the morning, though many clinicians prescribe it off-label for longer durations when benefits outweigh risks and this is clearly documented. 1, 2

Standard Dosing Regimen

Frequency:

• Once daily dosing is standard, taken approximately 2 hours after breakfast 3, 2
• Avoid late evening administration due to insomnia risk 1, 2
• Alternative low-dose regimen: 8 mg can be given up to 3 times daily, approximately 30 minutes before meals 1, 3

Dose Range:

• Start with 15 mg daily and titrate to 37.5 mg maximum based on response 3, 2
• Lower starting doses (8-9.375 mg) may be adequate for some patients 3
• The 8 mg formulation allows dosing as low as 4 mg using scored tablets 1

FDA-Approved Duration

The critical limitation is that phentermine is approved for only "a few weeks" of treatment, specifically up to 12 weeks (3 months). 1, 2 This is because:

• Phentermine is a Schedule IV controlled substance with abuse potential 1, 3
• Long-term safety data from controlled trials are lacking 1
• The FDA label explicitly states "short-term" use only 2

Off-Label Extended Use

Despite FDA approval for only 3 months, many practitioners prescribe phentermine for longer durations in off-label fashion when ongoing benefits are documented. 1, 3 When extending beyond 3 months:

• Document specific benefits, tolerance, and adverse effects at each visit 1
• Advise patients explicitly about off-label use and limited supporting data 1
• Verify state regulations, as some jurisdictions restrict duration of controlled substance prescriptions 1
• Monitor cardiovascular parameters (blood pressure and heart rate) periodically 1

Efficacy Assessment and Stopping Rules

Discontinue phentermine if inadequate weight loss is achieved:

• Stop if less than 5% weight loss at 12 weeks on maximum tolerated dose 3
• Research shows approximately 40% of 3-month non-responders may achieve ≥5% weight loss if continued to 6 months 4
• Conversely, about 10% of patients experience weight regain (potential tolerance) between 3-6 months 4

Duration in Clinical Trials

The evidence base shows variable treatment durations:

• Most obesity trials used phentermine for 12-24 weeks 1
• The CONQUER trial (phentermine-topiramate combination) extended to 56 weeks and showed sustained efficacy 5
• Real-world data demonstrates safe use for up to 2 years in selected patients 6

Special Populations

Renal impairment:

• Maximum 15 mg daily for severe renal impairment (eGFR 15-29 mL/min/1.73 m²) 2
• Avoid entirely if eGFR <15 mL/min/1.73 m² or on dialysis 2

Common Pitfalls to Avoid

• Do not prescribe beyond 3 months without explicit documentation of rationale, patient counseling about off-label use, and ongoing benefit assessment 1
• Do not continue if cardiovascular parameters worsen (increased blood pressure or heart rate) 1
• Do not use in patients with cardiovascular disease history, as this is an absolute contraindication regardless of duration 2
• Do not combine with MAOIs or within 14 days of MAOI discontinuation 1, 2