By Sumathi Reddy

There’s a surprising side effect of a marijuana habit that many people don’t know: Regular users may need more anesthesia during medical procedures to remain sedated.

As more states allow people to smoke pot and eat edibles legally, more doctors say they are asking about marijuana use—and urging honesty—before surgeries or procedures because habitual users may need more anesthesia and painkillers. In one study, people who reported they used cannabis required more anesthesia than people who didn’t use it.

“It’s incredibly important to disclose your cannabis use to your anesthesiologist,” says Staci Gruber, director of the Marijuana Investigations for Neuroscientific Discovery (MIND) program at McLean Hospital in Belmont, Mass., who co-wrote a recent study looking at interactions between cannabis and other drugs.

Hospitals already ask about alcohol consumption, which can also affect your anesthesia requirements, doctors say. Many people don’t readily mention marijuana use, though, and healthcare workers in some states don’t routinely ask.

A 2021 federal survey found that 18.7% of people age 12 or older had used marijuana in the past year.

It isn’t clear if the anesthesia complication is largely limited to people who use THC, the main psychoactive component of marijuana and what causes the high, or also extends to people who take cannabidiol, or CBD. The latter doesn’t produce the high that THC does and is sometimes used as a remedy for pain, sleep or anxiety. Many cannabis products such as edibles contain a combination of the two.

Julius Hyatt, an oral surgeon at the Maryland Center for Oral Surgery and Dental Implants in Cockeysville, Md., started noticing a few years ago that more of his patients were requiring larger amounts of anesthesia to remain sedated.

Some patients who were regular cannabis users needed two to three times the normal amount of the anesthetic propofol to stay under, he says. He and others in his practice also had to start adding other drugs sometimes used in sedation such as fentanyl and ketamine to maintain sedation for patients who were becoming restless during oral surgeries. Dr. Hyatt’s practice changed its health history form to ask more specific questions about patients’ marijuana use.

Doctors say you don’t need to worry about waking up during a procedure, though. Anesthesiologists monitor sedation levels and can give more if they notice signs that the drugs are wearing off. But regular marijuana users appear to need more anesthesia to become sedated and stay that way.

In January, the American Society of Regional Anesthesia and Pain Medicine published guidelines recommending that all patients undergoing anesthesia for an inpatient or outpatient procedure be screened and questioned about cannabis use, including how much they use and how often, how they take it, and when they last used it.

Researchers are still learning about how cannabis affects anesthesia, so the guidelines don’t specify how much more anesthesia users need or what amount of use requires changes.

Research on cannabis is scant, and often relies on self-reports, which aren’t always reliable. Most studies done examining the anesthesia issue have looked at propofol, a common anesthetic used in outpatient and inpatient procedures.

One of the largest studies, reviewing the records of people who have undergone procedures, examined 250 patients undergoing an endoscopy with anesthesia. The 25 who reported that they were cannabis users required a greater dose of anesthesia than the nonusers, according to the study, which was published in 2019.

A 2021 study found that cannabis use was associated with an increase in the propofol dose needed to sedate patients for an endoscopy. Daily users needed a higher dose than weekly or monthly ones, the study found.

Both CBD and THC, or tetrahydrocannabinol, are processed by an enzyme system in the liver, says Dr. Gruber. Other drugs, including anesthetics, are also processed in the liver, which may partly explain why cannabis use can interfere with sedation.

The way you consume cannabis makes a difference, she says. It has a longer-lasting impact when it is ingested, in say an edible, and has to be processed by the liver than when it is inhaled.

Cannabis can also interfere with painkillers after surgery, doctors say. A 2018 study looking at 261 patients from four trauma centers found that cannabis users had higher pain scores and consumed 25% to 37% higher quantities of opioids compared with nonusers.

David Dickerson, chair of the American Society of Anesthesiologists’ Committee on Pain Medicine, says all patients undergoing surgery are asked about cannabis use in the Chicago-area hospital system where he works.

The patients who may need higher doses of anesthetic or be more closely monitored are those he considers “regular users,” who are taking cannabis more days than not.

Jeffrey Galinkin, chairman of perioperative services and anesthesia at Medical Center of Aurora in Colorado, says he has been routinely asking patients who will get anesthesia about marijuana use for years. He follows it up by telling patients their answer will affect the amount of anesthetic and pain medication he will give them.

“When you say that, they become very truthful very quickly,” says Dr. Galinkin.