Phentermine Discontinuation Before Surgery
Stop phentermine at least 4 days (96 hours) before elective surgery requiring general anesthesia. 1
Evidence-Based Rationale
The 4-day discontinuation period is based on phentermine's pharmacokinetics and documented perioperative complications:
Phentermine has a half-life of approximately 19-24 hours, and a 4-day cessation allows for approximately 4-5 half-lives to elapse, ensuring near-complete drug elimination and minimizing perioperative autonomic complications 1
The American Gastroenterological Association specifically recommends discontinuing phentermine (and phentermine-containing medications) at least 4 days before procedures requiring anesthesia due to documented hyperadrenergic effects and catecholamine depletion risks 2
Documented Perioperative Risks
Phentermine poses specific anesthetic hazards that justify preoperative cessation:
Refractory hypotension is the most commonly reported complication during induction of general anesthesia, potentially unresponsive to indirect-acting vasopressors like ephedrine that rely on catecholamine release 1
Additional documented complications include hypertension, bradycardia, hyperthermia, cardiac depression, hypoglycemia, and acute pulmonary edema during the perioperative period 1
Case reports document perioperative hypertensive crises in patients taking phentermine, attributed to its sympathomimetic properties 3
As a norepinephrine reuptake inhibitor, phentermine can cause autonomic dysfunction and catecholamine depletion, creating unpredictable hemodynamic responses during anesthesia 2, 1
Clinical Context
This recommendation differs from older guidelines:
The 4-day cessation period is shorter than the classic 2-week discontinuation recommended for fenfluramine-phentermine ("fen-phen") combinations, reflecting phentermine monotherapy's distinct pharmacology 1
One older case report suggested 7 days of preoperative discontinuation, but this was based on limited evidence and has been superseded by pharmacokinetic-based recommendations 3
Critical Implementation Points
Cardiovascular monitoring is essential: Blood pressure and heart rate should be monitored closely intraoperatively for signs of autonomic instability, even after appropriate discontinuation 1
Direct-acting vasopressors may be required: If hypotension occurs, phenylephrine or norepinephrine may be more effective than ephedrine due to potential catecholamine depletion 1
Patient counseling is mandatory: Inform patients of the increased surgical risk if phentermine is not discontinued appropriately, and document this shared decision-making 1
Elective surgery should be postponed if the patient has taken phentermine within 4 days of the scheduled procedure, unless the surgical urgency outweighs the anesthetic risks 1
Common Pitfall
Do not assume phentermine is safe to continue perioperatively simply because it is commonly prescribed—with 2.43 million prescriptions written annually in the United States, many patients present for surgery while taking this medication, but documented complications justify routine preoperative cessation 1