What is the usage and guidance for Phentermine (Adipex) 37.5MG Oral Capsule for weight loss?

Phentermine 37.5 mg Oral Capsule for Weight Loss

Phentermine 37.5 mg should be prescribed as a short-term adjunct (a few weeks, up to 3 months) to comprehensive lifestyle modification including caloric restriction, exercise, and behavioral therapy for patients with BMI ≥30 kg/m² or BMI ≥27 kg/m² with weight-related comorbidities, but must be avoided in patients with cardiovascular disease. 1

Patient Selection Criteria

Appropriate candidates include:

• BMI ≥30 kg/m² OR BMI ≥27 kg/m² with comorbidities (hypertension, type 2 diabetes, dyslipidemia, obstructive sleep apnea) 2
• Younger patients without active or unstable coronary disease 3
• Patients without uncontrolled hypertension 3
• Patients who need appetite suppression assistance 3

Absolute contraindications:

• History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled hypertension) 1
• Current or recent (within 14 days) MAOI use 1
• Hyperthyroidism 1
• Glaucoma 1
• Pregnancy or nursing 1
• History of drug abuse 1
• Agitated states or anxiety disorders 1

Pre-Treatment Assessment

Before prescribing, obtain:

• Baseline blood pressure and heart rate 4
• Cardiovascular disease screening 4
• Pregnancy status in women of reproductive potential 4
• Thyroid function assessment to exclude hyperthyroidism 4
• Renal function (eGFR) 1
• Medication review for MAOIs and other contraindicated drugs 4

Dosing Regimen

Standard dosing:

• Start with 15 mg once daily in the morning, approximately 2 hours after breakfast 3, 1
• Can titrate to 37.5 mg once daily based on efficacy and tolerability 3, 1
• Lower starting doses (8-9.375 mg) may be appropriate for some patients 3
• Avoid late evening dosing due to insomnia risk 3, 1

Renal impairment adjustments:

• Severe renal impairment (eGFR 15-29 mL/min/1.73 m²): Maximum 15 mg daily 1
• eGFR <15 mL/min/1.73 m² or dialysis: Avoid use 1

Duration of Treatment

FDA-approved duration:

• Phentermine is approved for short-term use only (a few weeks, up to 3 months) 1, 3
• It is a Schedule IV controlled substance 2, 3
• Many practitioners prescribe off-label for longer durations, though long-term safety data are limited 3

Monitoring and Efficacy Assessment

Follow-up schedule:

• Monitor efficacy and safety monthly for the first 3 months, then at least every 3 months 2
• Check blood pressure and heart rate periodically 4

Discontinuation criteria:

• If ≤5% weight loss at 12 weeks, discontinue and consider alternative medications 2, 3
• If new symptoms of dyspnea, angina pectoris, syncope, or lower extremity edema develop (concern for primary pulmonary hypertension) 1

Expected Weight Loss

Efficacy data:

• Mean weight loss of 5.1% at 28 weeks compared to placebo 2
• In a 28-week trial with 15 mg daily: 46% achieved ≥5% weight loss and 20.8% achieved ≥10% weight loss 3
• At 37.5 mg daily, weight loss ranges from 6.6-7.4% 2

Common Adverse Effects

Most frequent side effects:

• Dry mouth 2, 3
• Insomnia/difficulty sleeping 2, 3
• Dizziness 2, 3
• Irritability 2, 3
• Constipation 2, 3
• Elevated heart rate and blood pressure 2

Critical Safety Warnings

Cardiovascular concerns:

• In patients with cardiovascular disease, do not prescribe phentermine; use lorcaserin or orlistat instead 2
• Phentermine causes mild increases in blood pressure and heart rate through sympathetic nervous system activation 2, 4
• Rare cases of primary pulmonary hypertension have been reported 1
• Rare cases of serious regurgitant cardiac valvular disease have been reported 1

Drug interactions:

• Contraindicated with MAOIs (risk of hypertensive crisis) 1
• Avoid concomitant alcohol use 1
• May alter insulin or oral hypoglycemic requirements in diabetic patients 1
• May decrease hypotensive effect of adrenergic neuron blocking drugs 1

Tolerance and dependence:

• Tolerance to anorectic effect typically develops within a few weeks 1
• If tolerance develops, discontinue medication rather than exceeding recommended dose 1
• Risk of abuse and dependence exists; prescribe the least amount feasible 1

Combination Therapy Considerations

Avoid combinations:

• Do not combine phentermine with other weight loss drugs (prescribed, over-the-counter, or herbal products) 1
• Safety and efficacy of combination therapy have not been established 1
• Do not combine with serotonergic agents such as SSRIs 1

Patient Counseling Points

Essential information to convey:

• Phentermine is only effective when combined with intensive lifestyle modification (diet, exercise, behavioral therapy) 2, 1
• Medication should not be used alone 2
• FDA approval is for short-term use only (3 months) 3, 1
• Report immediately any chest pain, shortness of breath, fainting, or leg swelling 1
• Keep medication in a safe place to prevent theft or misuse 1
• Avoid activities requiring alertness until response to medication is known 1

Clinical Pitfalls to Avoid

Common errors:

• Prescribing to patients with undiagnosed or inadequately controlled cardiovascular disease 2
• Failing to assess pregnancy status in women of reproductive potential 4
• Continuing therapy beyond 12 weeks without documented ≥5% weight loss 2, 3
• Prescribing without concurrent intensive lifestyle intervention 2, 1
• Exceeding recommended doses when tolerance develops 1
• Combining with other weight loss medications 1