What are the contraindications for phentermine and what patient education should be provided?

Phentermine Contraindications and Patient Education

Absolute Contraindications

Phentermine must never be prescribed to patients with any history of cardiovascular disease, including coronary artery disease, stroke, arrhythmias, congestive heart failure, or uncontrolled hypertension. 1, 2

Additional absolute contraindications include:

• Pregnancy or inadequate contraception in women of childbearing potential—phentermine is FDA pregnancy category X and contraindicated throughout pregnancy 1, 3
• Current MAOI use or within 14 days of discontinuation due to risk of hypertensive crisis 1, 3
• Hyperthyroidism (untreated)—additive sympathomimetic effects may precipitate arrhythmias 1, 4
• Glaucoma (particularly angle-closure type)—sympathomimetic agents worsen intraocular pressure 1, 4
• Agitated states or history of drug abuse—phentermine is a Schedule IV controlled substance with amphetamine-like properties 1, 3
• Concomitant use with other sympathomimetic amines 1
• Severe renal impairment (eGFR <15 mL/min/1.73 m²) or end-stage renal disease requiring dialysis 3

Relative Contraindications and Cautions

Patients with controlled hypertension (systolic <140 mmHg, diastolic <90 mmHg on non-MAOI antihypertensives) may be considered for phentermine with intensive monitoring, as clinical trials show blood pressure often decreases during treatment secondary to weight loss. 2

• Moderate renal impairment (eGFR 15-29 mL/min/1.73 m²)—limit dosage to 15 mg daily 3
• Elderly patients—start at low end of dosing range due to decreased renal, hepatic, and cardiac function 3
• Diabetes mellitus—insulin or oral hypoglycemic requirements may decrease; close glucose monitoring required 1, 3

Pre-Treatment Evaluation

Before prescribing phentermine, complete the following assessment:

• Cardiovascular screening: Obtain baseline blood pressure and heart rate 2, 4
• BMI verification: Confirm BMI ≥30 kg/m² or BMI ≥27 kg/m² with weight-related comorbidities 1, 4
• Pregnancy screening: Verify negative pregnancy status in all women of reproductive potential 1, 4
• Medication review: Screen for MAOI use within past 14 days, other sympathomimetics, and alcohol use 1, 4
• Medical history: Exclude hyperthyroidism, glaucoma, cardiovascular disease, psychiatric disorders (anxiety, agitation), and substance use disorders 1, 4
• Renal function: Assess serum creatinine/eGFR, especially in diabetic patients 4

No routine laboratory monitoring (CBC, metabolic panel, lipids, TSH) is mandated by phentermine guidelines; focus remains on cardiovascular parameters and weight-loss efficacy. 2

Dosing and Administration

• Standard dosing: 15-37.5 mg orally once daily in the morning to minimize insomnia 1, 2
• Low-dose option: 8 mg tablets can be dosed up to three times daily, with scored tablets allowing doses as low as 4 mg 2
• Timing: Morning administration is standard, though some clinicians use afternoon dosing to target high-risk eating periods (expert opinion only) 2
• Food: Absorption is not significantly affected by food 2

Duration of Treatment and Monitoring

FDA approval is limited to short-term use (3 months), but many experienced clinicians prescribe phentermine off-label for 3-6 months or longer based on efficacy and cardiovascular safety rather than arbitrary time limits. 2

Continuation Criteria

Continue phentermine if:

• ≥5% weight loss achieved after 12 weeks 1, 2
• No cardiovascular contraindications develop 2
• Blood pressure and heart rate remain controlled 2
• Medication is well-tolerated without serious adverse effects 2

Discontinuation Criteria

Stop phentermine if:

• <5% weight loss after 12 weeks on maximum dose 1, 2
• Development of cardiovascular disease or uncontrolled hypertension 2
• Intolerable side effects that don't resolve with dose reduction 2
• Pregnancy occurs or is planned 2

Monitoring Protocol

• Blood pressure and heart rate at every clinical visit throughout treatment 1, 2
• Weight assessment monthly for first 3 months, then at least every 3 months 1
• No mandatory washout period required between treatment courses if discontinued for non-safety reasons 2

Common Side Effects

Inform patients to expect:

• Central nervous system: Overstimulation, restlessness, dizziness, insomnia, irritability, headache 1, 3
• Gastrointestinal: Dry mouth, unpleasant taste, constipation, diarrhea 1, 3
• Cardiovascular: Palpitations, tachycardia, modest blood pressure elevation 1, 3
• Other: Euphoria, dysphoria, tremor 3

Critical Patient Education Points

Emphasize that phentermine monotherapy should not be confused with the discontinued "fen-phen" combination—the valvular heart disease and pulmonary hypertension associated with fen-phen were caused by fenfluramine, not phentermine. 2

Key Counseling Messages

• Pregnancy prevention: Women of childbearing potential require reliable contraception and potentially monthly pregnancy testing during extended use 2
• Alcohol avoidance: Concomitant alcohol use may result in adverse drug reactions 3
• Medication interactions: Report all medications, especially MAOIs, other stimulants, and diabetes medications 3
• Cardiovascular symptoms: Immediately report chest pain, palpitations, shortness of breath, or severe headache 2
• Psychiatric effects: Monitor for mood changes, anxiety, or agitation 3
• Abuse potential: Phentermine is a Schedule IV controlled substance; use only as prescribed 3
• Off-label duration: FDA approval is for 3 months, but continuation beyond this period is common based on clinical response 2

Safer Alternatives for High-Risk Patients

For patients with cardiovascular disease or significant cardiovascular risk factors, recommend orlistat (lipase inhibitor) or lorcaserin as safer alternatives without sympathomimetic activity. 1

• GLP-1 receptor agonists (semaglutide 2.4 mg, liraglutide 3.0 mg) provide superior weight-loss efficacy (≈21% with tirzepatide at 72 weeks vs. ≈5.1% with phentermine at 28 weeks) and favorable cardiovascular safety profiles 1, 2
• Orlistat is appropriate for patients with cardiovascular contraindications, though it has lower efficacy (≈3.1% weight loss at 1 year) 1

Common Pitfalls to Avoid

• Do not prescribe phentermine to patients with "mild" hypertension without first achieving blood pressure control <140/90 mmHg 2
• Do not combine phentermine with other weight-loss medications outside FDA-approved fixed-dose combinations (e.g., phentermine/topiramate ER) 2
• Do not continue phentermine beyond 12 weeks if <5% weight loss achieved—consider alternative therapy 1, 2
• Do not overlook pregnancy screening in women of reproductive age—this is an absolute contraindication 1, 4
• Do not confuse phentermine safety profile with historical fen-phen concerns 2