What are the guidelines for using phentermine (anorexiant) in patients with a history of cardiovascular disease, glaucoma, or hyperthyroidism?

Phentermine Use in Patients with Cardiovascular Disease, Glaucoma, or Hyperthyroidism

Phentermine is absolutely contraindicated in patients with any history of cardiovascular disease (including coronary artery disease, stroke, arrhythmias, or congestive heart failure), glaucoma, or hyperthyroidism—do not prescribe this medication to these patients under any circumstances. 1, 2, 3

Absolute Contraindications

Cardiovascular Disease

• Any history of cardiovascular disease is an absolute contraindication, including: 3

  • Coronary artery disease
  • Stroke
  • Arrhythmias (including atrial fibrillation) 4
  • Congestive heart failure
  • Uncontrolled hypertension

• The FDA drug label explicitly lists "history of cardiovascular disease" as a contraindication, and this is reinforced by the American Gastroenterological Association guidelines, which state that phentermine should be avoided in patients with cardiovascular disease due to lack of large cardiovascular outcome trial data. 1, 2, 3

• Phentermine acts as a norepinephrine-releasing agent that activates the sympathetic nervous system, causing potential cardiovascular effects including palpitations, tachycardia, elevated blood pressure, and ischemic events. 1

Hyperthyroidism

• Untreated hyperthyroidism is an absolute contraindication to phentermine use. 1, 3

• The American Gastroenterological Association explicitly states that phentermine should not be used in patients with untreated hyperthyroidism due to concerns for arrhythmias and seizures. 1

• Thyroid function must be assessed and documented as normal before initiating phentermine therapy. 2, 5

Glaucoma

• Glaucoma is an absolute contraindication to phentermine use. 1, 3

• The FDA label and clinical guidelines contraindicate phentermine in glaucoma due to sympathomimetic effects that can precipitate acute angle-closure glaucoma. 2, 5, 3

• Patients at high risk for glaucoma should also avoid phentermine. 1

Critical Pre-Prescribing Algorithm

Before prescribing phentermine, you must systematically screen for these absolute contraindications: 2, 5

1. Cardiovascular assessment: Obtain complete cardiovascular history including any remote history of arrhythmias, structural heart disease, coronary disease, stroke, or heart failure 2

2. Thyroid screening: Verify thyroid function is normal; hyperthyroidism must be excluded 2, 5

3. Ophthalmologic history: Screen for glaucoma or risk factors for acute angle-closure glaucoma 2, 5

4. Blood pressure measurement: Document baseline blood pressure; uncontrolled hypertension is a contraindication 2, 5

Common Pitfalls to Avoid

• Failing to obtain a complete cardiovascular history is a critical screening error—you must specifically ask about remote history of arrhythmias or structural heart disease, not just recent events. 2

• Inadequate thyroid function assessment before initiation is a critical error that can lead to serious arrhythmias and seizures. 2

• Confusing phentermine monotherapy with "fen-phen": The discontinued phentermine-fenfluramine combination was associated with valvular heart disease and pulmonary hypertension, but these risks were attributed to fenfluramine, not phentermine. However, the FDA label notes that the possibility of an association between valvular heart disease and phentermine alone cannot be ruled out. 5, 3

Alternative Medications for These Patients

If weight loss pharmacotherapy is needed in patients with these contraindications, consider: 5

• GLP-1 receptor agonists (semaglutide 2.4 mg or liraglutide 3.0 mg) have superior efficacy and different safety profiles without the cardiovascular risks of sympathomimetic agents 5

• Orlistat works by inhibiting gastrointestinal lipases rather than affecting the sympathetic nervous system, making it safer for patients with cardiovascular concerns 5