What is the recommended workup for a patient considering phentermine (Adipex-P) for weight loss?

Phentermine Pre-Treatment Workup

Before prescribing phentermine, screen for cardiovascular disease, measure baseline blood pressure and heart rate, assess for contraindications including pregnancy risk, and evaluate renal function. 1, 2

Cardiovascular Assessment

Screen for active or unstable cardiovascular disease, as phentermine is absolutely contraindicated in patients with coronary artery disease, stroke history, arrhythmias, congestive heart failure, or uncontrolled hypertension. 2

• Obtain baseline blood pressure and heart rate measurements, as phentermine causes mild increases in both parameters through sympathetic nervous system activation 1, 3
• Assess for history of cardiovascular events or symptoms (chest pain, palpitations, syncope) 2
• Even mild hypertension requires caution, as blood pressure may increase further 2
• Younger patients without active coronary disease are more appropriate candidates 1

Contraindication Screening

Phentermine has multiple absolute contraindications that must be ruled out before prescribing: 1, 2

• Pregnancy status: Phentermine is contraindicated in pregnancy and all female patients of reproductive potential require counseling 1
• Hyperthyroidism: Must be excluded before starting therapy 1, 2
• Glaucoma: Contraindicated due to sympathomimetic effects 1, 2
• Agitated states or anxiety disorders: May be exacerbated by phentermine's stimulant properties 1, 2
• History of drug abuse: Phentermine is a Schedule IV controlled substance with abuse potential 2
• MAOI use: Contraindicated during or within 14 days of MAOI administration due to risk of hypertensive crisis 1, 2
• Concurrent sympathomimetic amine use: Should not be combined with other stimulants 1, 2

Renal Function Assessment

Evaluate renal function with estimated glomerular filtration rate (eGFR), as phentermine undergoes substantial renal excretion: 2

• Limit dosage to 15 mg daily maximum for severe renal impairment (eGFR 15-29 mL/min/1.73 m²) 2
• Avoid use entirely in patients with eGFR <15 mL/min/1.73 m² or end-stage renal disease requiring dialysis 2

Metabolic and Comorbidity Evaluation

Assess for diabetes and obtain baseline glucose measurements, as insulin or oral hypoglycemic requirements may decrease with weight loss: 2

• Document baseline weight and calculate BMI to confirm indication (BMI ≥30 kg/m² or ≥27 kg/m² with comorbidities) 2
• Measure waist circumference for tracking metabolic improvements 4
• Screen for sleep disorders or insomnia, as phentermine may worsen these conditions 1

Patient Counseling and Documentation

Counsel patients that phentermine is FDA-approved only for short-term use (3 months), though off-label longer duration is common in clinical practice: 1

• Discuss that phentermine should be used as adjunct to reduced-calorie diet, exercise, and behavioral modification 2
• Warn about common side effects: dry mouth, difficulty sleeping, dizziness, and irritability 1, 3
• Advise against late evening administration to minimize insomnia risk 2
• Counsel about potential for tolerance development within a few weeks 2
• Discuss that concomitant alcohol use may result in adverse drug reactions 2
• Establish plan for monitoring response: discontinue if inadequate weight loss achieved 1

Baseline Laboratory and Vital Signs

Obtain the following baseline measurements for comparison during treatment: 3

• Blood pressure and heart rate 1, 3
• Fasting glucose (especially in diabetic or prediabetic patients) 2
• Weight and BMI 2
• Renal function (eGFR/creatinine) 2