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Study Finds Cannabis Use Impacts Anesthesia & Opioid Use


There is a growing body of research that cannabis is an effective treatment for chronic pain. But a new study points out that cannabis users may need more anesthesia during surgery – and opioids afterward.

Imagine this. You’ve broken your leg and will need surgery. You’re also a regular cannabis user, but decide not to mention this to your doctor. Maybe you live in a state without access to medical marijuana – or maybe you just don’t want to be stereotyped by the medical establishment. 

Either way, according to a new study, omitting this information may make pain relief harder to come by in the hospital. 

This week, at the annual meeting of the American Society of Anesthesiologists, doctors discussed the impact of cannabis use on the amount of pain relief patients need when they are in the hospital for surgery. 

In a statement to CNN, lead author Dr. Ian Holmen, an anesthesiology resident at the University of Colorado Hospital in Aurora, explained: 

“There is some evidence that cannabis may be beneficial for chronic and nerve pain. However, early research suggests that this is not the case for acute pain such as for surgery of a broken leg.”

The as-yet-unpublished study compared the pain relief outcomes of 118 patients who needed surgery for a broken leg. Of these, 30 patients reported using cannabis. What researchers found was a marked difference in pain outcomes, backing up previous research on the subject.

In short, patients that used cannabis required higher doses of sevoflurane, an inhaled anesthetic that keeps you asleep during a procedure. Not only that, researchers found that patients who use cannabis also reported higher pain levels after surgery. And they needed nearly 60% more opioid painkillers per day while recuperating in the hospital.

Holmen explained the findings:

“It’s similar to flying a plane. You have a takeoff section, and then you have your cruising section and then your landing. In anesthesia these are induction, maintenance, and emergence,” he said. “We found that it’s not just in the induction phase of anesthesia that you need more anesthesia, but even during that cruising phase you need more inhaled anesthetic.”

The authors of the study do not suggest that people shouldn't be using cannabis. They do, however, stress the need for clear communication with physicians, especially before surgery.

So what are the implications for cannabis users who need surgery? Generally, it’s just about finding a good dose of anesthetic during surgery, and pain relief after. But for those with heart or lung health issues, the situation becomes more serious according to  Dr. David Dickerson, vice-chair of the ASA’s Committee on Pain Medicine: 

“Sevoflurane (the anesthetic used during surgery) has a very clear dose-dependent effect on blood pressure,” said Holmen. “The more sevoflurane you receive in the OR, the more a patient’s blood pressure drops. If you have heart problems or lung problems coming into the OR, it could be dangerous.”

Why exactly cannabis use has this impact on patients in acute pain is not clear from the information gained during the study. But there are some possible explanations, according to Dickerson. One possibility is that cannabis changes the way a person’s nervous system responds to pain and to painkillers:

“Is there a change in our neurophysiology or our nervous system that causes us to be in a more excited state, that then causes worsened pain after an injury or during surgical healing, or that actually increases the amount of anesthesia that takes us to sleep or deeper than sleep?” 

Another possibility has to do with the way medications are metabolized: 

“Cannabis is metabolized in the liver. Medications like anesthetics and our pain medicines are also metabolized in the liver,” said Dickerson. “Is there a change in the way the liver’s metabolic function is occurring after being exposed to cannabis?”

In either case, both Dickerson and Holmen agree that more research is needed on how cannabis interacts with anesthesia. 

Another (pretty major) element of this research that would be worth looking into is the definition of “cannabis use.” The study did not gather information about the types of cannabis products the patients used and neither were the amount, method, or frequency of use looked into. These variables obviously require further research.

And just to be clear, the authors of the study are not suggesting that people shouldn’t be using cannabis. Rather, according to Holmen:

“This study shows that it is important for patients to tell their physician anesthesiologist if they have used cannabis products prior to surgery to ensure they receive the best anesthesia and pain control possible, including the use of non-opioid alternatives.” 

In other words, if you’re a regular cannabis user, make sure that your doctors are informed before going into surgery.

“We want to know there might be a need for more anesthesia,” says Dickerson. “The last thing we want to do is to be under-dosing if someone is going to have an increased requirement. The more information we have, the more we can react and monitor to keep a patient safe during a procedure.”

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