Cannabis and Anesthesia: The Drugs Don’t Work?

A widely reported study found that patients who had used cannabis in the month prior to fracture surgery required more anesthesia during the op and had more pain afterward (1,2).

Ian Holmen, the lead author of the study and an anesthesiology resident at the University of Colorado Hospital, Aurora, USA, says, “Myself and several of my colleagues had noticed that some patients who reported cannabis use were reacting differently to anesthesia (often requiring more anesthesia than we thought was normal for their other past medical history or age) and seemed to require more pain medications after surgery.” 

Holmen and colleagues Jeffrey Paul Beach, Alex Kaizer, and Ramakrishna Gumidyala set out to test their hypothesis by reviewing the dose of anesthetic drugs received during surgery by cannabis users and non-users. There was little difference for most drugs, but cannabis users did receive significantly more of an inhaled anesthetic, sevoflurane. Cannabis users also reported higher pain scores post-surgery and received more opioids. 

It's hard to say why cannabis use might affect pain perception given the huge number of active compounds found in the plant, says Holmen, but THC has many receptors in the central and peripheral nervous system. “Some of these receptor locations are involved in pain modulation which impacts our perception of pain such as pain intensity and our emotional response to pain,” says Holmen. “It is possible that when a cannabis user stops using cannabis [before surgery] that these receptors are subsequently not bound and consequently the perception of pain is worse.” 

Holmen is clear that this retrospective review study has a number of limitations. No data was collected on the amount or frequency of cannabis use. Plus, as a related commentary article points out, anesthesiologists knew who had used cannabis and it’s possible they gave these patients more sevoflurane in the expectation they would require it (3).

Previous studies of pain sensitization in cannabis users have returned mixed results, with one recent clinical study showing no increased sensitivity to pain in regular cannabis users (4), while an earlier study found the reverse (5). 

“It is clear that there needs to be significantly more research on the topic and that there is still a dearth of high-quality studies,” concludes Holmen. However, he says there is one important take-home message from the study: “It is important for patients to disclose to their anesthesia provider if they use marijuana recreationally before surgery so that they can receive the best anesthetic and multimodal pain management.”