What is the typical dosing of Phentermine (phentermine) for weight loss?

Phentermine Dosing for Weight Loss

The standard dosing of phentermine for weight loss is 15-30 mg once daily, taken approximately 2 hours after breakfast, with a typical duration of therapy being short-term (a few weeks). 1

Dosing Recommendations

• Phentermine is FDA-approved as a short-term (a few weeks) adjunct for weight management in patients with BMI ≥30 kg/m² or ≥27 kg/m² with risk factors such as hypertension, diabetes, or hyperlipidemia 1
• The usual adult dose is 15-30 mg taken once daily, approximately 2 hours after breakfast for appetite control 1
• Administration of one 30 mg capsule daily has been found adequate to suppress appetite for 12-14 hours 1
• Late evening medication should be avoided due to the risk of insomnia 1
• For patients with severe renal impairment (eGFR 15-29 mL/min/1.73 m²), the maximum dosage should be limited to 15 mg daily 1

Clinical Considerations

• Phentermine is available in various formulations including capsules (15 mg, 30 mg, 37.5 mg) and tablets (8 mg, 37.5 mg) 2
• Phentermine is a Schedule IV controlled substance due to its potential for abuse, though the primary concerns with long-term use relate to cardiovascular effects rather than addiction 2
• Common side effects include dry mouth, insomnia, dizziness, irritability, constipation, and headache 2
• Phentermine should be avoided in patients with cardiovascular disease, uncontrolled hypertension, hyperthyroidism, glaucoma, agitated states, history of drug abuse, or during/within 14 days of taking MAOIs 1

Monitoring and Efficacy

• Blood pressure and heart rate should be monitored periodically during treatment 2
• When combined with lifestyle interventions, phentermine can achieve approximately 5-10% body weight loss in successful patients 2
• In clinical trials, phentermine (15 mg daily) was associated with 6.0 kg weight loss at 28 weeks compared to placebo 2

Combination Therapy

• For patients requiring combination therapy, phentermine/topiramate extended-release (Qsymia) is available in four dose strengths: 3.75/23 mg, 7.5/46 mg, 11.25/69 mg, and 15/92 mg 3, 4
• Qsymia follows a specific dose escalation schedule: starting with 3.75/23 mg daily for 14 days, then increased to 7.5/46 mg daily, with potential further titration if needed 4
• Treatment with Qsymia should be discontinued if 3% weight loss is not achieved after 12 weeks at 7.5/46 mg daily or if 5% weight loss is not achieved after 12 weeks at the maximum 15/92 mg daily dose 3, 4

Important Caveats

• Phentermine is only approved for short-term use (typically 12 weeks), though many clinicians prescribe it for longer durations as off-label therapy 2
• The safety and efficacy of combination therapy with phentermine and other weight loss medications (except for FDA-approved combinations like Qsymia) have not been established 1
• Phentermine may impair the ability to engage in potentially hazardous activities such as operating machinery or driving 1
• A reduction in dose of insulin or oral hypoglycemic medication may be required in some patients taking phentermine 1