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Warning: Cannabis Can Mess with Going Under

Dragana Komnenov PhD
4 surgeons operating under a spotlight

Cannabis consumption just prior to surgery can mean you need more anesthetic when under.

It’s almost two decades ago that Dr. Symons, a UK anesthesiologist at Barnet General Hospital told a story about a patient, in his 30’s, who appeared healthy pre-operation. The patient was there for the removal of wisdom teeth, which is normally a routine procedure. When under anesthesia, however, things were different.



Initial induction of inspired anesthetic ran smoothly. But, the anesthetist noticed that the patient was not settling, even though he had a normal respiratory pattern. The doctor gave additional boluses of the volatile anesthetic, as well as an intravenous anesthetic in order to fully induce anesthesia. The procedure went well, and Dr. Symons went to visit the patient after he woke up. After Dr. Symons told the patient that it was very difficult to induce and maintain anesthesia, the patient reluctantly disclosed that he had smoked cannabis the night before. The anesthetist advised him refrain from smoking cannabis pre-op in the future.

Why did this happen? What mechanisms could be at play to interfere with anesthesia in individuals who smoke cannabis? Firstly, due to the slow elimination of cannabinoids from the body, they could remain present in the tissue for days or weeks. As such, cannabinoids could interfere with the action of the anesthetic.

Secondly, cannabinoids exert complex actions on seizure activity, whereby they have been known to be proconvulsant in some cases and anticonvulsant in other cases. Additionally, it is now recommended that a corticosteroid, such as dexamethasone be given to patients who smoke cannabis pre-operatively. This further underlines the importance of disclosing cannabis smoking.

Lack of Evidence for Either side

The majority of the clinical evidence describing effects of cannabis on general anesthesia and in the perioperative setting are anecdotal case reports. We still lack concrete evidence arguing for either side.

In fact, some of cannabis’ effects such as sedation, bronchodilation, dryness of respiratory secretions, vein dilation, and increase in heart rate without inducing increase in blood pressure actually make it a great option for pre-medication.

Likewise, relieving nausea and its analgesic potential, without affecting respiratory depression make it an attractive candidate as a post-operative drug. In order to shed some light on this issue, investigators in Israel initiated a clinical trial to evaluate the use of cannabis in a perioperative setting. The agent they are using is Nabiximols (Sativex®), which contains THC and CBD in a 1:1 ratio.

The trial is still ongoing, and the investigators just published their first finding regarding anesthesia depth. The depth of anesthesia is routinely measured by the bispectral index (BIS) monitoring, which measures the electrical activity of the brain under the sedative effects of anesthesia. Computer-generated BIS scores range from 0 to 100, with 0 being coma, 40–60:general anesthesia, 60–90: sedation, and 100 being awake.

Anesthesia mask coming toward the camera

“a shallower depth of anesthesia” when under

The investigators found that indeed, the patients who were receiving 21.6mg THC and 20mg CBD had higher BIS values. This was even under same dose of anesthesia. According to the BIS score scale, this would indicate a shallower depth of anesthesia. However, the investigators suggest that the higher BIS score may indicate a different pattern of brain electrical activity. Rather, anyway, than shallower anesthesia. This hypothesis still needs exploration.

According to the available data, cannabinoids affect those under anesthesia in one of two ways. First, by frankly decreasing the depth of anesthesia. Second, by altering the brain wave patterns such that it appears through BIS monitoring that the anesthesia is shallower when it may not be. In any event, disclosing cannabis use with the doctor and anesthetist is likely a good idea, since there are some prophylactic measures already in place for individuals using it (such as dexamethasone administration).

Dragana Komnenov
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