What are the contraindications for phentermine (Adipex-P)?

Contraindications for Phentermine (Adipex-P)

Phentermine is absolutely contraindicated in patients with untreated hyperthyroidism, history of cardiovascular disease, uncontrolled hypertension, glaucoma, agitated states, history of drug abuse, and during or within 14 days of monoamine oxidase inhibitor use. 1, 2, 3

Absolute Contraindications

• Cardiovascular Conditions:

  • History of cardiovascular disease
  • Uncontrolled hypertension
  • Advanced arteriosclerosis
  • Pulmonary hypertension
  • Valvular heart disease

• Endocrine Conditions:

  • Untreated hyperthyroidism (due to risk of arrhythmias and seizures)

• Neuropsychiatric Conditions:

  • Agitated states or anxiety disorders
  • History of drug abuse
  • Glaucoma (risk of acute angle-closure glaucoma)
  • Seizure disorders

• Medication Interactions:

  • Use during or within 14 days of monoamine oxidase inhibitor (MAOI) therapy (risk of hypertensive crisis)

• Special Populations:

  • Pregnancy (Category X) and breastfeeding
  • Pediatric patients (safety not established)

Relative Contraindications/Use with Caution

• Mild hypertension (risk of blood pressure elevation)
• Diabetes mellitus (may require adjustment of insulin or oral hypoglycemic medications)
• Renal impairment (dose adjustment required; avoid in severe renal impairment with eGFR <15 mL/min/1.73 m²)
• Elderly patients (start at lower doses due to potential for decreased renal function)
• Concomitant use with alcohol (may result in adverse drug reactions)

Medication Interactions Requiring Caution

• Insulin and oral hypoglycemic medications: May require dose adjustment 3
• Adrenergic neuron blocking drugs: Phentermine may decrease their hypotensive effect 3
• Tramadol: Severe potential drug-drug interaction 4
• Dextromethorphan: Moderate potential drug-drug interaction 4

Monitoring Requirements

1. Before initiating therapy:

   • Assess cardiovascular status including blood pressure and heart rate
   • Check thyroid function (TSH and free T4)
   • Screen for glaucoma
   • Evaluate for history of substance abuse

2. During therapy:

   • Regular monitoring of blood pressure and heart rate
   • In patients with diabetes: monitor blood glucose levels
   • In patients with renal impairment: monitor renal function

Important Considerations

• Phentermine is FDA-approved only for short-term use (≤12 weeks) 2
• Phentermine is classified as a Schedule IV controlled substance due to potential for abuse 3
• Long-term use has been associated with pharmacological tolerance, dependence, and withdrawal symptoms 2
• Rare but serious adverse events have been reported with phentermine use, including ischemic colitis 5 and atrial fibrillation 6

By thoroughly screening patients for contraindications before prescribing phentermine and monitoring appropriately during treatment, clinicians can help minimize the risk of adverse events while maximizing the potential benefits of this weight management medication.