Phentermine Should Be Discontinued At Least 4 Days Before Anesthesia
Phentermine must be discontinued at least 4 days prior to any procedure requiring general anesthesia due to significant perioperative risks including refractory hypotension, hypertension, bradycardia, and hyperthermia. 1
Perioperative Risks of Phentermine
Phentermine poses several specific anesthetic risks that make its discontinuation before surgery essential:
Cardiovascular Complications
- Sympathomimetic effects: As a monoamine sympathomimetic, phentermine elevates norepinephrine levels in the central nervous system 1
- Hemodynamic instability: Can cause both hypertensive complications and paradoxical hypotension during anesthesia 1
- Refractory hypotension: Most concerning is hypotension that may be unresponsive to vasopressors that rely on catecholamine release (like ephedrine) 2
- Bradycardia: Can occur during anesthesia in patients taking phentermine 2
Other Perioperative Complications
- Hyperthermia: Body temperature dysregulation has been reported 2
- Autonomic dysfunction: Phentermine's effect as a norepinephrine reuptake inhibitor can lead to catecholamine depletion 1
- Cardiac depression: Can occur during anesthesia 2
- Acute pulmonary edema: Has been reported as a complication 2
Discontinuation Protocol
The evidence clearly supports a specific discontinuation timeline:
- Minimum discontinuation period: 4 days before surgery 1, 2
- Rationale: Based on the half-life of phentermine to allow adequate clearance from the body
- Note: This differs from the classic 2-week discontinuation period that was recommended for "fen-phen" (fenfluramine-phentermine combination) 2
Special Considerations
Elective vs. Emergency Surgery
- For elective procedures: Always discontinue phentermine at least 4 days prior
- For emergency procedures: Inform anesthesiologist about phentermine use, as special anesthetic management may be required
Anesthetic Management if Phentermine Cannot Be Discontinued
If surgery must proceed without adequate phentermine washout:
- Direct-acting vasopressors (like phenylephrine) may be more effective than indirect-acting agents
- Careful monitoring of blood pressure, heart rate, and body temperature is essential
- Skilled anesthesiologist should be prepared for autonomic instability
Post-Surgical Considerations
- Phentermine should not be restarted until the patient has fully recovered from anesthesia and surgical stress
- Monitor for cardiovascular stability before resuming phentermine
Risk Factors for Complications
Patients with these conditions may have increased risk of perioperative complications with phentermine:
- History of cardiovascular disease
- Uncontrolled hypertension
- Untreated hyperthyroidism
- Concurrent use of monoamine oxidase inhibitors (MAOIs)
The evidence strongly supports discontinuing phentermine before anesthesia to prevent potentially serious complications that could increase morbidity and mortality during surgical procedures.