What are the guidelines for prescribing Phentermine to patients with a history of cardiovascular disease, glaucoma, or hyperthyroidism?

Phentermine Prescribing Guidelines for High-Risk Patients

Phentermine is absolutely contraindicated in patients with any history of cardiovascular disease, untreated hyperthyroidism, or glaucoma, and should never be prescribed to these populations. 1, 2

Absolute Cardiovascular Contraindications

Phentermine must be avoided entirely in patients with: 3, 1, 2

• Coronary artery disease (including remote history) 1, 2
• Stroke or cerebrovascular disease 1, 2
• Arrhythmias (including past episodes) 1, 2
• Congestive heart failure 1, 2
• Uncontrolled hypertension 3, 2

The FDA labeling explicitly lists "history of cardiovascular disease" as a contraindication, and the American Gastroenterological Association reinforces this due to lack of large cardiovascular outcome trial data for long-term use. 3, 1, 2 Even patients with controlled hypertension require extreme caution, as phentermine activates the sympathetic nervous system and can elevate blood pressure and heart rate. 3, 4

Critical pitfall: Do not confuse phentermine monotherapy with the discontinued "fen-phen" combination—the valvular heart disease and pulmonary hypertension risks were attributed to fenfluramine, not phentermine. 4 However, rare cases of primary pulmonary hypertension and serious regurgitant cardiac valvular disease have been reported with phentermine alone. 2

Hyperthyroidism - Absolute Contraindication

Phentermine is contraindicated in patients with untreated hyperthyroidism due to concerns for arrhythmias and seizures. 3, 1, 2

Before prescribing phentermine: 1, 4

• Assess thyroid function to exclude hyperthyroidism 1, 4
• Document thyroid status in the medical record 1
• Do not prescribe until hyperthyroidism is adequately treated and controlled 3, 2

The combination of phentermine's sympathomimetic effects with the hypermetabolic state of hyperthyroidism creates unacceptable cardiovascular risk. 3

Glaucoma - Absolute Contraindication

Phentermine is contraindicated in patients with glaucoma, particularly closed-angle glaucoma, due to its sympathomimetic effects. 3, 1, 2

The American Academy of Ophthalmology states that glaucoma is a contraindication due to sympathomimetic effects that can increase intraocular pressure. 4 This contraindication is particularly critical when phentermine is combined with topiramate, which carries additional risk of acute angle-closure glaucoma. 3, 5

Additional Absolute Contraindications

Phentermine must not be prescribed to patients with: 3, 2

• Current or recent MAOI use (within 14 days) - risk of hypertensive crisis 3, 1, 5, 2
• Pregnancy - teratogenic concerns 3, 1, 5, 2
• History of drug abuse - Schedule IV controlled substance with abuse potential 3, 1, 5, 2
• Agitated states or anxiety disorders - may be exacerbated by stimulant properties 3, 4, 2

Mandatory Pre-Treatment Screening Algorithm

Before prescribing phentermine, clinicians must: 1, 4, 5

1. Obtain complete cardiovascular history, including remote history of arrhythmias or structural heart disease 1, 5
2. Measure baseline blood pressure and heart rate 1, 4, 5
3. Assess thyroid function to exclude hyperthyroidism 1, 4, 5
4. Screen for glaucoma history 4, 5
5. Evaluate renal function (eGFR) - limit dosage to 15 mg daily for severe renal impairment (eGFR 15-29 mL/min/1.73 m²); avoid use if eGFR <15 mL/min/1.73 m² 5, 2
6. Assess pregnancy risk in all females of reproductive potential and provide contraception counseling 4, 5
7. Screen for substance abuse history 1, 2
8. Review all current medications for MAOI use or other sympathomimetic amines 3, 1, 2

Failing to obtain a complete cardiovascular history or inadequate assessment of thyroid function before initiation are critical screening errors. 1

Monitoring Requirements for Appropriate Candidates

If phentermine is prescribed to patients without contraindications: 4, 5

• Monitor blood pressure and heart rate at every visit throughout treatment 3, 4, 5
• Discontinue if inadequate response: If patient has not lost at least 5% of body weight after 12 weeks on maximum dose 4, 5
• Counsel on common side effects: dry mouth, difficulty sleeping, dizziness, irritability, constipation 3, 4
• Document off-label use if prescribing beyond FDA-approved 12 weeks 4

Safer Alternatives for High-Risk Patients

For patients with cardiovascular concerns, hyperthyroidism, or glaucoma who require weight management: 4

• GLP-1 receptor agonists (semaglutide 2.4 mg or liraglutide 3.0 mg) have superior efficacy and do not carry sympathomimetic cardiovascular risks 4
• Orlistat works by inhibiting gastrointestinal lipases rather than affecting the sympathetic nervous system, making it safer for patients with cardiovascular risk factors 3, 4