Phentermine/Topiramate Dosing for Weight Loss
Starting Dose and Initial Titration
Begin with phentermine/topiramate ER 3.75 mg/23 mg once daily for 14 days, then increase to 7.5 mg/46 mg once daily as the standard maintenance dose. 1, 2
- This two-week initiation period allows for gradual tolerance development before reaching the therapeutic maintenance dose 1
- The 7.5 mg/46 mg dose produces an average weight loss of 7.8% of baseline body weight compared to 1.2% with placebo 1, 2
Dose Escalation Protocol
If weight loss is less than 3% after 12 weeks on the 7.5 mg/46 mg dose, escalate to 11.25 mg/69 mg daily for 14 days, followed by the maximum dose of 15 mg/92 mg daily. 1
- The maximum 15 mg/92 mg dose achieves approximately 9.8% weight loss compared to 1.2% with placebo 1, 2
- At the maximum dose, 70% of patients achieve ≥5% weight loss and 48% achieve ≥10% weight loss 3
- Discontinue the medication if less than 5% weight loss is achieved after 12 weeks at the maximum dose 1, 2
Patient Eligibility Criteria
This medication is indicated for adults with: 1, 2
- BMI ≥30 kg/m², OR
- BMI ≥27 kg/m² with weight-related comorbidities (hypertension, type 2 diabetes, dyslipidemia)
Mandatory Pre-Treatment Screening
Absolute Contraindications to Identify
- Uncontrolled hypertension (systolic >140 mmHg or diastolic >90 mmHg) 1
- Active coronary artery disease or recent cardiovascular events 1
- Glaucoma, particularly closed-angle type 1
- Pregnancy or inadequate contraception in women of childbearing potential 1, 2
Required Baseline Assessments
- Obtain a negative pregnancy test in all women of childbearing potential before initiating therapy 1, 2
- Measure baseline blood pressure and heart rate 1, 2
- Check serum bicarbonate level to identify pre-existing metabolic acidosis 1
- Counsel about teratogenic risk (orofacial clefts with first-trimester exposure) and need for reliable contraception 1, 2
Monitoring Schedule and Parameters
Visit Frequency
Parameters to Monitor at Each Visit
- Blood pressure and heart rate (phentermine may cause modest tachycardia) 1, 2
- Serum bicarbonate periodically to detect metabolic acidosis 1
- Weight and efficacy assessment 1
- Repeat pregnancy tests as clinically indicated 1
Common Adverse Effects to Anticipate
The most frequent side effects include: 1, 2
- Paresthesias (≈23%)
- Dry mouth (≈19%)
- Headache (≈16%)
- Insomnia (≈10%)
- Constipation and dizziness
Critical Discontinuation Protocol
If discontinuation is necessary, taper gradually by taking one capsule every other day for at least one week before stopping completely—never stop abruptly to avoid precipitating seizures. 1
Additional Therapeutic Benefits Beyond Weight Loss
Treatment produces improvements in: 1
- Waist circumference reduction
- Systolic and diastolic blood pressure decrease
- Hemoglobin A1c improvement in patients with diabetes