What is the recommended dosing regimen for Qsymia (phentermine/topiramate) in a patient who is a candidate for this medication?

Qsymia Prescribing: Recommended Dosing Regimen

Start Qsymia at 3.75 mg phentermine/23 mg topiramate once daily in the morning for 14 days, then increase to 7.5 mg/46 mg daily; evaluate weight loss at 12 weeks and escalate to the maximum dose of 15 mg/92 mg daily if less than 3% weight loss is achieved, discontinuing if less than 5% weight loss occurs after 12 weeks at maximum dose. 1, 2

Mandatory Titration Schedule

The dose escalation must follow this specific protocol to minimize adverse events:

• Days 1-14: 3.75 mg phentermine/23 mg topiramate once daily in the morning 1, 2
• Day 15 onward: Increase to 7.5 mg phentermine/46 mg topiramate once daily 1, 2
• Week 12 evaluation: Assess weight loss at the 7.5/46 mg dose 1, 2
  • If <3% weight loss: Escalate to 11.25 mg/69 mg daily for 14 days, then increase to maximum dose of 15 mg/92 mg daily 1, 2, 3
  • If ≥3% weight loss: Continue at 7.5/46 mg dose 2
• Discontinuation rule: Stop treatment if <5% weight loss after 12 weeks at the maximum 15/92 mg dose 1, 2, 3

Patient Selection Criteria

Qsymia is FDA-approved for patients meeting these specific criteria:

• BMI ≥30 kg/m² without comorbidities, OR 1
• BMI ≥27 kg/m² with at least one weight-related comorbidity (type 2 diabetes, hypertension, dyslipidemia) 1

Critical Contraindications and Monitoring

Absolute Contraindications

• Pregnancy: Topiramate causes cleft lip/palate and other fetal malformations 1, 2
  • Obtain β-HCG before initiating, followed by monthly self-testing at home 1
  • Require effective contraception in all women of childbearing potential 1, 2, 3
• Glaucoma (particularly acute angle-closure) 1
• Hyperthyroidism 1
• Use of MAOIs within 14 days 1
• History of cardiovascular disease or uncontrolled hypertension 1

Mandatory Monitoring Parameters

• Electrolytes and creatinine: Check before treatment and during therapy due to risk of metabolic acidosis and hypokalemia 1, 3
• Heart rate: Monitor for tachycardia, as phentermine increases sympathetic tone 1
• Mental health: Screen for depression, anxiety, and suicidal ideation at each visit 1
• Blood glucose: In diabetic patients on insulin or sulfonylureas, monitor closely for hypoglycemia 1, 3

Expected Weight Loss Outcomes

Clinical trial data demonstrates dose-dependent efficacy:

• 7.5/46 mg dose: 7.8% weight loss at 1 year; 62% of patients achieve ≥5% weight loss 2, 4
• 15/92 mg dose: 9.8% weight loss at 1 year; 70% of patients achieve ≥5% weight loss 2, 4
• Placebo comparison: The higher dose produces approximately 8.8 kg greater weight loss than placebo at 56 weeks 4

Common Adverse Effects to Counsel Patients About

The most frequent side effects include:

• Paresthesia (tingling sensations): 14-21% of patients 1, 4
• Dry mouth (xerostomia): 13-21% 1, 4
• Constipation: 15-17% 1, 4
• Dysgeusia (altered taste): 7-10% 1, 4
• Insomnia: 6-10% 1, 4
• Cognitive impairment: Concentration and memory difficulties 1

Critical Prescribing Pitfalls to Avoid

• Never discontinue the 15/92 mg dose abruptly: Taper over at least 1 week to prevent seizures 1, 3
• Do not split or divide the once-daily dose: Qsymia must be taken once daily in the morning, not in divided doses 1, 2
• Avoid combining with other sympathomimetic amines: Risk of additive cardiovascular effects 1
• Adjust dose in renal or hepatic impairment: Maximum dose should not exceed 7.5/46 mg in moderate renal impairment 1
• Discontinue at least 4 days before general anesthesia: Risk of perioperative complications 3

Drug Interactions Requiring Dose Adjustments

• Concurrent use with diuretics (especially HCTZ or furosemide): Monitor for hypokalemia 1
• Oral contraceptives: May cause menstrual spotting due to altered estrogen/progestin metabolism 1
• Strong CYP3A inhibitors (itraconazole, ketoconazole, clarithromycin): May require dose reduction 3