How long should phentermine (anorexiant) be held prior to anesthesia?

Phentermine Discontinuation Prior to Anesthesia

Phentermine should be discontinued at least 4 days before any procedure requiring anesthesia to minimize perioperative cardiovascular complications. 1

Rationale for Discontinuation

Phentermine is a sympathomimetic medication commonly prescribed for weight loss that poses specific risks during anesthesia:

• Mechanism of action: Acts as a norepinephrine reuptake inhibitor and sympathomimetic agent 1
• Perioperative risks:
  • Hyperadrenergic effects during induction (hypertension)
  • Refractory hypotension during anesthesia (due to catecholamine depletion)
  • Autonomic dysfunction
  • Other reported complications include bradycardia, hyperthermia, and cardiac depression 2

Evidence-Based Recommendation

The 2022 American Gastroenterological Association (AGA) clinical practice guideline specifically addresses this issue, stating that "phentermine (and phentermine-containing medications) be discontinued at least 4 days before a procedure requiring anesthesia" 1. This recommendation is based on:

• Phentermine's half-life and pharmacokinetic properties
• Case reports documenting perioperative complications
• Systematic reviews identifying specific anesthetic risks

Important Considerations

• Vasopressor response: Phentermine-induced hypotension may be unresponsive to vasopressors that rely on catecholamine release (like ephedrine) 2
• Common misconception: The 4-day discontinuation period differs from the classic 2-week discontinuation period that was recommended for "fen-phen" (fenfluramine-phentermine combination) 2
• Alternative recommendation: Some sources suggest discontinuing sympathomimetic weight loss medications for at least 7 days before surgery 3, but the most recent guideline evidence supports the 4-day timeframe 1

Special Situations

• Bariatric surgery patients: While low-dose phentermine has been studied for preoperative weight loss in bariatric surgery patients 4, it should still be discontinued 4 days before the actual surgical procedure
• Long-term users: Patients taking phentermine long-term (>12 weeks) may have additional risks due to potential drug-drug interactions with anesthetic medications 5

Perioperative Management Algorithm

1. Identify phentermine use during preoperative evaluation
2. Discontinue phentermine at least 4 days before scheduled procedure
3. Inform anesthesia team about recent phentermine use
4. Monitor closely for:
   • Blood pressure instability
   • Body temperature changes
   • Signs of autonomic dysfunction
5. Be prepared for potential refractory hypotension that may not respond to standard vasopressors

This approach prioritizes patient safety by minimizing the risk of potentially serious cardiovascular complications during anesthesia while following the most current evidence-based recommendations.