Why To Stop Phentermine Before A Colonoscopy

In our latest question and answer, the pharmacist discusses why you should stop phentermine (Adipex-P) before a colonoscopy.


Hi Brian, I just read your article on phentermine and how it's recommended that patients stop usage prior to surgery. What is your recommendation on stopping phentermine prior to endoscopy procedures, i.e. colonoscopy/EGD procedures where nurses are administering Versed and Fentanyl as conscious sedation to these patients? Should it be stopped for these patients as well, or not? Thoughts?

Asked by Geo On Jan 30, 2019

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Jan 31, 2019
Colonoscopy On Blackboard


It has been well documented that phentermine, a prescription weight-loss drug with similar effects to amphetamines, should be stopped before surgery involving general anesthesia.

The concern is that the combination could put you at significant risk of dangerous side effects. We have written a number of articles on this subject, one of which is available here:

As a quick summation, having phentermine in your system while undergoing general anesthesia puts you more at risk for:

  • Heart attack and cardiac arrest
  • Low blood pressure
  • High blood pressure

Low blood pressure, also known as hypotension, is thought to be the most significant risk. However, on the opposite end of the spectrum, cases of high blood pressure (hypertension) have been reported as well.

How long before your procedure you should stop phentermine will vary on a case by case basis and by the physician. In general, it should be stopped at least four days before surgery, with recommendations ranging up to 3 weeks before. Be sure to follow the directions of your doctor.

When it comes to surgery that does not involve 'general anesthesia,' recommendations are a bit vaguer.

Nevertheless, there is a significant amount of data, and case reports of adverse reactions, that show phentermine should be stopped before a colonoscopy procedure as well.

Stopping Phentermine Before Colonoscopy

Phentermine Molecular Structure

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.

Other Considerations

Regarding the 'conscious sedation' drugs you mentioned, fentanyl and Versed (midazolam), the precautions with phentermine don't apply since they have different effects from general anesthesia.

Nevertheless, phentermine could potentially offset some of their sedative effects (since it is a stimulant after all).

You may also see drug interactions come up for phentermine and fentanyl, with the warning of serotonin syndrome. The FDA in 2016 warned about combining opioid drugs, like fentanyl, and other drugs that could increase serotonin.

This particular interaction most likely isn't of much concern since phentermine has weak effects on serotonin and fentanyl is being administered as a one-time dose. Therefore, the precaution about phentermine use and colonoscopies is limited to the procedure itself. 

Section Summary

There are several reasons to stop phentermine before your colonoscopy procedure, including the fact that it can put you at risk for ischemic colitis (decreased blood flow to the colon).

What Is Phentermine?

Since I am discussing the use of phentermine concerning medical procedures, it makes sense to do a quick overview of the drug.

Phentermine is a stimulant, chemically similar to amphetamines like Adderall. It is indicated for the short-term treatment of obesity in those 17 years and older.

It most likely works via a variety of mechanisms. Pharmacologically, it releases norepinephrine and dopamine and inhibits their reuptake. This results in a speeding up of your metabolism and decreases appetite. It may also raise serotonin levels, but mildly.

It is important to avoid dosing phentermine in the evening, as it may cause you to have trouble sleeping.

  • Elsevier ClinicalKey: Phentermine (subscription required) (Accessed 1/31/19)
  • Transmural Colonic Infarction after Routine Colonoscopy in a Young Patient without Risk Factors. PubMed
  • Phentermine: A Systematic Review for Plastic and Reconstructive Surgeons. PubMed
  • Anesthetic considerations for the new antiobesity medications. PubMed
  • Phentermine and anaesthesia. PubMed
  • Ischemic colitis after weight-loss medication. PubMed
  • Amphetamine-related ischemic colitis causing gastrointestinal bleeding. PubMed
  • Ischemic Colitis. PubMed
  • Transmural Colonic Infarction after Routine Colonoscopy in a Young Patient without Risk Factors PubMed
  • Colonoscopy-induced ischemic colitis in patients without risk factors. PubMed
  • Post Colonoscopy Ischaemic Colitis in a Patient without Risk Factors for Colon Ischemia: A Case Report. PubMed
  • Pharmacologically mediated colon ischemia. PubMed

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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