What is the starting dose of phentermine (anorexiant) for an adult patient with a body mass index (BMI) of 30 or higher, or 27 or higher with at least one weight-related condition, for weight loss?

Starting Dose of Phentermine for Weight Loss

Start phentermine at 15 mg once daily in the morning, approximately 2 hours after breakfast, and titrate up to 30-37.5 mg daily based on tolerability and response. 1

FDA-Approved Dosing Strategy

The FDA label specifies that dosage should be individualized to obtain adequate response with the lowest effective dose, with the usual adult dose ranging from 15 mg to 30 mg daily 1. The medication should be administered approximately 2 hours after breakfast for appetite control, and late evening dosing must be avoided due to insomnia risk 1.

Initial Dosing Options

• Standard starting dose: 15 mg once daily in the morning 2, 1
• Alternative starting dose: 30 mg once daily (one capsule) has been found adequate for appetite suppression lasting 12-14 hours 1
• Lower starting doses available: Some patients may benefit from starting as low as 8-9.375 mg daily and titrating upward 2
• Low-dose formulation (Lomaira): Can be prescribed at 8 mg up to 3 times daily, approximately 30 minutes before meals 2

Dose Titration Algorithm

Week 1-2: Start with 15 mg once daily in the morning 2, 1

Week 3-4: If well-tolerated but inadequate appetite suppression, increase to 30 mg once daily 2

Week 5-12: May increase to maximum dose of 37.5 mg once daily if needed for additional effect 2, 1

Alternative split-dosing: 18.75 mg twice daily (half tablet) can be used instead of once-daily dosing 2

Response Assessment and Discontinuation Criteria

• Evaluate weight loss at 12 weeks on the current dose 2
• Discontinue if <5% weight loss achieved after 12 weeks on maximum tolerated dose 3, 2
• The American Gastroenterological Association recommends this 5% threshold as the minimum clinically meaningful weight loss 4

Special Dosing Considerations for Renal Impairment

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

Duration of Treatment

• FDA-approved duration: Short-term use only (up to 3 months) due to lack of long-term safety data 2, 1
• Off-label extended use: Many experienced clinicians prescribe for 3-6 months or longer, as obesity is now understood as a chronic metabolic disease requiring long-term management 3, 2
• If prescribing beyond 12 weeks, document specific benefits, tolerance profile, and counsel patients regarding off-label use and limited long-term safety data 3

Evidence Supporting Starting Dose Selection

A prospective multi-center study of 932 obese Mexican adults found that 30 mg phentermine was more effective than 15 mg at 3 months for anthropometric improvements, but both doses showed similar efficacy at 6 months 5. This supports starting at the lower 15 mg dose to minimize adverse effects, with titration to 30 mg if needed for enhanced early response.

Research demonstrates that phentermine 15 mg can be effective, with one case report showing successful weight loss when slowly up-titrated from 15 mg to 37.5 mg daily over time, resulting in 13% body weight reduction 6.

Common Pitfalls to Avoid

• Do not confuse phentermine monotherapy with "fen-phen": The discontinued phentermine-fenfluramine combination was associated with valvular heart disease and pulmonary hypertension—these risks were attributed to fenfluramine, not phentermine 3
• Avoid late evening dosing: This commonly causes insomnia due to phentermine's stimulant properties 2, 1
• Do not exceed recommended doses: Tolerance to anorectic effects typically develops within weeks; increasing beyond maximum dose is not recommended 1
• Screen for cardiovascular contraindications: History of cardiovascular disease, uncontrolled hypertension, coronary artery disease, stroke, or arrhythmias are absolute contraindications 3, 1

Pre-Treatment Requirements

Before prescribing phentermine, the American Gastroenterological Association recommends:

• Cardiovascular assessment: Measure baseline blood pressure and heart rate, as phentermine causes mild increases through sympathetic activation 3, 7
• Pregnancy screening: Phentermine is contraindicated in pregnancy; assess pregnancy status in all women of reproductive potential 3, 1
• Medication review: Ensure no concurrent MAOI use (contraindicated during or within 14 days of MAOI administration) 3, 1
• Screen for contraindications: Hyperthyroidism, glaucoma, agitated states, and history of drug abuse are absolute contraindications 3, 1

Expected Weight Loss Outcomes

• Mean weight loss: 5.1% at 28 weeks compared with placebo 2
• 46% of patients achieve ≥5% weight loss at 28 weeks on 15 mg daily 2
• 20.8% of patients achieve ≥10% weight loss at 28 weeks 2

Monitoring During Treatment

• Blood pressure and heart rate: Monitor periodically throughout treatment due to sympathomimetic effects 3, 7
• Common adverse effects to monitor: Dry mouth, insomnia, dizziness, irritability, constipation, and palpitations 2
• Discontinue immediately if: New unexplained dyspnea, angina pectoris, syncope, or lower extremity edema develop (rare cases of primary pulmonary hypertension reported) 1