How to manage a patient on Phentermine (Adipex)?

Managing a Patient on Phentermine (Adipex)

Phentermine must be used only as a short-term adjunct (a few weeks, maximum 3 months per FDA approval) combined with an intensive lifestyle program including caloric restriction, exercise, and behavioral modification—never as monotherapy. 1

Indications and Patient Selection

• Prescribe phentermine only for patients with BMI ≥30 kg/m² or BMI ≥27 kg/m² with weight-related comorbidities (hypertension, type 2 diabetes, dyslipidemia, obstructive sleep apnea). 2
• Absolutely contraindicated in patients with cardiovascular disease (coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled hypertension)—switch to orlistat or lorcaserin instead. 2, 1

Pre-Treatment Assessment

Before prescribing phentermine, you must: 3

• Screen for cardiovascular disease and measure baseline blood pressure and heart rate 3
• Assess pregnancy status in all women of reproductive potential—phentermine is absolutely contraindicated in pregnancy and nursing 3, 1
• Screen for hyperthyroidism, glaucoma, agitated states/anxiety disorders, and history of substance abuse—all are contraindications 3, 1
• Verify the patient is not taking MAOIs or has not taken them within the past 14 days—risk of hypertensive crisis 3, 1
• Evaluate renal function (limit to 15 mg daily if eGFR 15-29 mL/min/1.73m²; avoid entirely if eGFR <15 or on dialysis) 1

Dosing Strategy

• Start with 15 mg orally once daily in the morning (or consider 8 mg up to three times daily for afternoon/evening eating patterns). 2, 1, 4
• Maximum dose is 37.5 mg daily, though most guidelines recommend 15 mg daily as standard. 2, 1
• Avoid late evening administration due to insomnia risk. 1
• Can be taken with or without food. 1

Monitoring Protocol

• Assess efficacy and safety monthly for the first 3 months, then at least every 3 months thereafter. 2
• Monitor blood pressure and heart rate at each visit—even mild hypertension requires caution as phentermine causes sympathetic activation. 3, 5
• Discontinue phentermine if <5% weight loss is achieved after 12 weeks on maximum tolerated dose—consider alternative medications (orlistat, lorcaserin, naltrexone/bupropion, liraglutide). 2, 3

Common Side Effects to Counsel Patients About

• Central nervous system: dizziness, dry mouth, difficulty sleeping, irritability, headache, restlessness 2, 5, 1
• Gastrointestinal: nausea, vomiting, diarrhea, constipation 2, 5
• Cardiovascular: palpitations, tachycardia, elevated blood pressure 5, 1

Critical Safety Warnings

• Rare but serious: primary pulmonary hypertension—discontinue immediately if new dyspnea, angina, syncope, or lower extremity edema develops. 1
• Rare but serious: regurgitant cardiac valvular disease has been reported. 2, 1
• Tolerance typically develops within a few weeks—do not exceed recommended dose; instead, discontinue the medication. 2, 1
• Risk of abuse and dependence (Schedule IV controlled substance)—prescribe the least amount feasible to minimize overdosage risk. 1
• Warn patients to avoid alcohol due to potential adverse drug reactions. 1
• May impair ability to operate machinery or drive—counsel accordingly. 1

Drug Interactions to Avoid

• MAOIs (during or within 14 days): absolute contraindication due to hypertensive crisis risk 1, 6
• Other sympathomimetic amines: contraindicated 2, 1
• Tramadol: severe potential drug-drug interaction (most commonly co-prescribed in the severe category) 6
• Insulin and oral hypoglycemics: may require dose reduction as weight loss improves glycemic control 2, 1
• Adrenergic neuron blocking drugs: phentermine may decrease their hypotensive effect 1

Special Populations

• Type 2 diabetes patients: Consider GLP-1 analogues (e.g., liraglutide) instead, which reduce hyperglycemia in addition to promoting weight loss. 2
• Cardiovascular disease patients: Phentermine is contraindicated—use orlistat or lorcaserin as safer alternatives. 2
• Geriatric patients: Use with caution due to substantial renal excretion. 1
• Renal impairment: Limit to 15 mg daily if eGFR 15-29; avoid if eGFR <15 or dialysis-dependent. 1

Duration of Therapy

• FDA-approved only for short-term use (3 months), though off-label longer duration is common in clinical practice—weigh benefits against cardiovascular risks, abuse potential, and tolerance development. 3, 1
• Expected weight loss: approximately 5.1% at 28 weeks compared to placebo. 2

Key Clinical Pitfalls

• Never prescribe phentermine alone without concurrent intensive lifestyle intervention—it is ineffective as monotherapy. 2, 1
• Do not combine with other weight loss medications—safety and efficacy of combinations not established (except FDA-approved phentermine/topiramate ER formulation). 1
• Do not continue beyond 12 weeks if inadequate response (<5% weight loss)—this indicates treatment failure. 2, 3
• Prescribe minimal quantities to reduce abuse, diversion, and overdose risk. 1