Phentermine Discontinuation Before Anesthesia
Phentermine should be discontinued at least 4 days before any procedure requiring anesthesia. 1, 2, 3
Rationale for the 4-Day Washout Period
The American Gastroenterological Association clinical practice guideline specifically mandates stopping phentermine and phentermine-containing medications at least 4 days prior to procedures requiring anesthesia. 1, 2 This recommendation is based on phentermine's pharmacologic properties as a sympathomimetic norepinephrine reuptake inhibitor, which creates dual perioperative cardiovascular risks:
- Hyperadrenergic effects that can cause hypertension, tachycardia, and hyperthermia during anesthesia 2, 3
- Paradoxical refractory hypotension during surgery, likely due to catecholamine depletion and autonomic dysfunction 1, 2, 3
The 4-day discontinuation period is notably shorter than the classic 2-week period historically recommended for fenfluramine-containing combinations, reflecting phentermine's specific half-life and pharmacokinetic profile. 3
Documented Perioperative Complications
Multiple case reports have documented serious anesthetic complications with perioperative phentermine use, including: 3
- Refractory hypotension (most commonly reported)
- Hypertension
- Bradycardia
- Cardiac depression
- Hyperthermia
- Acute pulmonary edema
- Hypoglycemia
Critical caveat: Phentermine-induced hypotension may be unresponsive to vasopressors that rely on catecholamine release, such as ephedrine, requiring direct-acting vasopressors instead. 3
Cardiovascular Monitoring Requirements
Patients who have been taking phentermine require enhanced perioperative monitoring: 3
- A skilled anesthesiologist should monitor intraoperative blood pressure continuously
- Body temperature monitoring for signs of autonomic dysfunction
- Blood pressure and heart rate should be monitored closely, especially in the first 12 weeks after resumption of phentermine postoperatively 1
Patient Counseling
The patient should be informed about the increased surgical risk associated with perioperative phentermine use, and the decision to proceed with elective surgery in a patient taking phentermine should be made with caution. 3 The postoperative period provides an opportunity to reassess the need for continued phentermine therapy, particularly given that phentermine is FDA-approved only for short-term use (≤12 weeks). 1
Additional Contraindications
Phentermine should be avoided entirely in patients with: 4, 1
- History of cardiovascular disease
- Uncontrolled hypertension
- Current use of monoamine oxidase inhibitors (or within 14 days of discontinuation)
- Untreated hyperthyroidism