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Regular users of marijuana may need up to three times more anesthetic to keep them unconscious during surgical procedures, so they need to tell surgical personnel about it, say doctors who have studied the phenomenon. And a preliminary study suggests that cannabis users have more pain after surgery.

“Why this might be is, like anesthesia itself, something of a mystery,” reports The Economist. “The problem is magnified by a growing number of Americans getting high,” with recreational cannabis legal in 20 states. “A 2021 federal survey found that 18.7% of people age 12 or older had used marijuana in the past year,” The Wall Street Journal reports.

“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,” the Journal reports. “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.”

Published studies of the problem have been limited, mainly because federal law thwarts studies of cannabis. Studies have focused on endoscopy, a procedure in which a long tube with a lens and a light is passed down the throat to the digestive tract. A study in Ontario found that daily users of cannabis needed more profopol, a common surgical sedative, than weekly or monthly users.

“Smoking weed before surgery can make patients confused, irritable and sometimes even violent when they wake up,” The Economist reports. “A working paper presented in October, which has yet to be peer-reviewed, suggests that weed can hinder recovery, too. Of the nearly 35,000 Cleveland Clinic patients assessed, those who used marijuana within 30 days of their operation experienced 14% more pain in the day after surgery and took 7% more prescription opioids to ease it.” A study published in 2018 “found that cannabis users had higher pain scores and consumed 25% to 37% higher quantities of opioids compared with nonusers,” The Wall Street Journal reports.

The American Society of Regional Anesthesia and Pain Medicine said in January that pregnant women should be discouraged from smoking cannabis, “and non-emergency surgeries should be postponed by at least two hours if the patient comes in blitzed. A slew of other new reports warn anaesthesiologists of the mounting risks,” The Economist reports. “The common perception that marijuana eases nerves works against doctors. Some people seem to be getting stoned before they arrive at the hospital in order to calm themselves down. New users may be particularly likely to smoke for such a purpose. For a better experience, patients should forgo the parking-lot hit.”