Why Do You Need To Stop Phentermine Before Surgery?

Answer

Hello and thanks for reaching out to us! While recommendations will vary by the practitioner, phentermine is generally recommended to be discontinued at least 7 days before surgery.

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 of 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.

Hypotension

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

Phentermine & Colonoscopy Risks

Just as with surgeries involving general anesthesia (which puts you in an unconscious state), phentermine should be stopped before a colonoscopy, but for different reasons. Since a colonoscopy procedure doesn't involve general anesthesia, the reasons to stop are distinctly different.

The two primary reasons to stop are:

• Phentermine increases your risk of a serious complication known as 'ischemic colitis'.
• Phentermine delays 'gastric emptying', or how quickly the contents of your stomach move to the small intestine.

In the instructions given to you by your doctor, there is a good chance that phentermine is listed as a medication to stop before undergoing the procedure. Be sure to follow this advice.

Phentermine - Ischemic Colitis

Ischemic colitis is essentially a condition that results from a restriction of blood flow to the colon, which can result in both inflammation and injury. While ischemic colitis itself isn't uncommon, it occurring as a complication from a colonoscopy procedure is quite rare.

The side effects of phentermine are well-documented and include a narrowing of the blood vessels and increased blood pressure. It has also been associated explicitly with causing decreased blood flow in the colon and therefore, is a risk factor for causing ischemic colitis. There have been several case reports of phentermine, or similar stimulants, causing it.

One such case report, published in the Canadian Journal of Gastroenterology and Hepatology, states:

"This report describes a temporal association with the use of phentermine and the development of ischemic colitis. Heightened awareness and appropriate surveillance is warranted to determine whether the use of weight-loss drugs, such as phentermine, can lead to ischemic colitis."

Another study published in Baylor University Medical Center Proceedings gives a pretty firm conclusion regarding phentermine:

"Dietary supplementation with phentermine (an amphetamine-derived sympathomimetic) can be associated with ischemic colitis. Amphetamine-induced systemic effects are believed to be due to release of vasoactive amines."

Reports of colonoscopy-induced ischemic colitis are rare, and colonoscopies are often used as a diagnostic tool for them. Nevertheless, since phentermine is associated with complications, it is best to discontinue it before your procedure.

Delayed Gastric Emptying

Phentermine can cause delayed gastric emptying, which means it takes longer for food to leave your stomach.

One study, published in the British Journal of Clinical Pharmacology, found fenfluramine, a drug very similar to phentermine, delayed gastric emptying time by about 15%.

Since colonoscopies require a clear colon, there is a chance that phentermine could complicate your bowel evacuation procedures. This likely isn't a huge concern but stopping phentermine well-before your bowel-clearing medications would be prudent.

When To Stop Phentermine Before Surgery

Most sources recommend discontinuing phentermine at 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, discontinuing the drug at least one week before surgery is prudent. The half-life (the 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.

Personally, I have seen instructions to discontinue phentermine ranging from four days before surgery/colonoscopy procedures to 3 weeks before. Be sure to follow the instructions your doctor gives you.

What Is Phentermine?

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.

Final Words

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