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

Phentermine Dosing for Weight Loss

The recommended dosing for phentermine is 15 to 37.5 mg orally once daily in the morning, approximately 2 hours after breakfast, with FDA approval for short-term use (up to 3 months), though many practitioners prescribe it off-label for longer durations. 1

Standard Dosing Regimens

Start with 15 mg once daily in the morning and titrate upward based on response and tolerability, with a maximum dose of 37.5 mg daily. 2, 1

Specific Dosing Options:

• Low-dose initiation: Begin with 8-9.375 mg daily (quarter tablet) for patients who may be sensitive to stimulants or have cardiovascular concerns 3
• Intermediate dose: 15-18.75 mg daily (half tablet) provides a middle ground 3
• Standard dose: 15-30 mg once daily in the morning 2, 1
• Maximum dose: 37.5 mg once daily 2, 1
• Alternative low-dose formulation (Lomaira): 8 mg tablets can be dosed up to 3 times daily (approximately 30 minutes before meals) 2, 3

Avoid late evening dosing due to insomnia risk. 1

Renal Dosing Adjustments

For patients with severe renal impairment (eGFR 15-29 mL/min/1.73 m²), limit the maximum dose to 15 mg daily. 1

Avoid phentermine entirely in patients with eGFR <15 mL/min/1.73 m² or end-stage renal disease requiring dialysis. 1

Duration of Treatment

Phentermine is FDA-approved only for short-term use (up to 12 weeks) as a Schedule IV controlled substance. 2, 3, 1

However, many practitioners prescribe phentermine for longer durations off-label when patients continue to benefit and tolerate the medication well. 2, 3 In clinical practice, treatment extending to 6 months has shown continued efficacy, with approximately 40% of 3-month non-responders achieving ≥5% weight loss by 6 months. 4

Efficacy Assessment and Discontinuation Criteria

Evaluate weight loss at 12 weeks: discontinue phentermine if the patient has not achieved at least 5% weight loss from baseline. 3

Expected Weight Loss:

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

Patient Selection

Ideal Candidates:

• Younger patients without active cardiovascular disease 2, 3
• Patients with controlled or no hypertension 2, 3
• Those needing appetite suppression assistance 2, 3
• Patients with higher baseline hunger and lower cognitive restraint (these characteristics predict better response) 5

Avoid in These Patients:

• History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, heart failure, uncontrolled hypertension) 1
• Anxiety disorders or insomnia (phentermine may exacerbate these conditions) 2
• Hyperthyroidism 2, 1
• Glaucoma 2, 1
• History of drug abuse 2, 1
• Current or recent MAOI use (within 14 days) 2, 1
• Pregnancy or breastfeeding 2, 1

Common Adverse Effects

The most frequent side effects include dry mouth, insomnia, dizziness, irritability, constipation, elevated blood pressure, elevated heart rate, and palpitations. 2, 3

Serious but rare risks include primary pulmonary hypertension and valvular heart disease (primarily reported with phentermine-fenfluramine combinations, but cannot be ruled out with monotherapy). 1

Critical Contraindications and Warnings

Do not combine phentermine with other weight loss medications, including over-the-counter products, herbal supplements, or serotonergic agents (SSRIs), as safety and efficacy of combination therapy have not been established. 1

Phentermine is classified as pregnancy category X—counsel all women of reproductive potential appropriately. 2

Long-term use may lead to pharmacological tolerance, dependence, and withdrawal symptoms. 2

Practical Prescribing Algorithm

1. Screen for absolute contraindications (cardiovascular disease, uncontrolled hypertension, hyperthyroidism, glaucoma, pregnancy, MAOI use) 1
2. Assess renal function and adjust dosing accordingly 1
3. Start with 15 mg once daily in the morning (or 8 mg if concerned about tolerability) 3, 1
4. Titrate upward after 2-4 weeks if inadequate response and good tolerability 3
5. Evaluate at 12 weeks: discontinue if <5% weight loss achieved 3
6. Monitor blood pressure and heart rate regularly during treatment 2
7. Consider discontinuation at 3 months per FDA labeling, or continue off-label if patient continues to benefit 2, 3, 1