Why You Need To Stop Phentermine Before Surgery

Before surgery, patients are instructed to stop certain medications for a period of time before their procedure. A common medication listed is phentermine (Adipex-P), a popular medication used to treat obesity and suppress appetite. In this article, we discuss why phentermine should be stopped before surgery and the precautions associated with it.


How long do I have to be off Phentermine before having surgery?

Asked by Ash On Dec 30, 2017

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Jan 04, 2018

While recommendations will vary by practitioner, phentermine is generally recommended to be discontinued at least 7 days before surgery.

Phentermine is classified as a stimulant medication, chemically similar to amphetamines. It works primarily by increasing the release of two different neurotransmitters, norepinephrine and dopamine, causing a net increase in their levels. In addition, phentermine is believed to affect the satiety center in the hypothalamus, "tricking" or "influencing" the body into decreasing appetite and lessening the urge to eat.


Phentermine & Anesthesia Risks

The primary concern with phentermine when undergoing surgical procedures is the potential interaction between the drug and anesthesia. Phentermine has a variety of effects on the body, many of which can interfere with or increase the effects of anesthesia. Below, we discuss some of the risks.

Cardiac Adverse Reactions

Studies show that the use anesthetics during surgery may sensitize the myocardium (the muscular tissue of the heart) to the effects of stimulant drugs such as amphetamine and phentermine. Sensitizing the myocardium could make the heart more susceptible to the effects of the increased neurotransmitters that result from phentermine administration. This could potentially increase the risk of cardiac arrest and heart attack.

Blood Pressure

Phentermine can raise both systolic and diastolic blood pressure in patients. When undergoing surgery, high blood pressure can increase the risk of hemorrhage as well as increase the risk of heart failure, stroke and heart attack.


Somewhat surprisingly, there are case reports in which patients on phentermine therapy experienced hypotension (low blood pressure) while under anesthesia. 

When To Stop Phentermine Before Surgery

Most sources recommend discontinuing phentermine as least one week prior to surgery. Many surgeons may recommend a longer period of time, with suggestions up to 2-3 weeks being fairly common.

When we take a look at the pharmacokinetic data on phentermine, discontinuning phentermine at least one week before surgery is prudent. The half life (time it takes for our bodies to metabolize 50% of a drug) is reported to range from 19-24 hours.

Generally, it is acknowledged that at least 5 half-lives need to elapse to consider a drug completely metabolized and eliminated from the body. This would equate to about 5 days with phentermine specifically. Therefore, the recommendation of discontinuing phentermine 7 days before surgery is prudent to allow for variations in individuals.

About the Pharmacist

Dr. Brian Staiger Pharm.D

Dr. Brian Staiger is a licensed pharmacist in New York State and the founder of PharmacistAnswers.com. He graduated from the University At Buffalo with a Doctor of Pharmacy degree in 2010. He has been featured in numerous publications including the Huffington Post as well as a variety of health and pharmacy-related blogs. Please feel free to reach out to him directly if you have any inquiries or want to connect! He's answered thousands of medication and pharmacy-related questions and he's ready to answer yours! Brian.Staiger@PharmacistAnswers.com Office: 716-389-3076

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