Cannabis can have many affects on the human body. Cannabis plants produce chemicals known as cannabinoids, which induces a wide range of effects on its consumers. At least 85 of these cannabinoids have been identified within the plant, of which the two richest ones are cannabidiol (CBD) and tetrahydrocannabinol (THC). THC is psychoactive (mind-altering), whereas CBD is not.

With the widespread use of cannabis in either or both forms for medicinal and recreational purposes, it is important to be open with your physician about your consumption.

What information should you tell your anesthesiologist? Here are some important questions you should answer.

1. How often do you consume cannabis?

2. Do you consume cannabis for recreational or medicinal purposes?

3. If medicinal, what condition are you seeking to treat?

4. How do you consume cannabis — do you vape, smoke or consume edibles?

5. Do you consume CBD? If so, how often?

Unless you are acutely intoxicated, it is unlikely that your case would be canceled but it is important to know the affects that cannabis can have on the body before, during, and after surgery. Most practitioners recommend that patients should avoid the use of cannabis at least 3–4 weeks before their surgery.

The inhalation of marijuana results in a peak effect of cannabinoids in just 15 minutes, which can last up to 4 hours. Smoking cannabis can have similar affects on the lungs as cigarettes. These affects can vary in patients but can be fatal. [1]

It can cause a cough, increased mucous production, and cause inflammation of the lungs. This inflammation can cause an asthmatic effect, which can sometimes lead to a deadly condition called bronchospasm. Bronchospasm is constriction of the muscles that line the airway leading to difficulty breathing. Cannabis consumption can also cause adverse breathing patterns in patients called hypoventilation.

Cannabis can have significant affects on the heart. One study showed that an elevated heart rate (tachycardia) could last for 90 minutes after the onset of marijuana inhalation[2]. Young patients who received higher doses were at risk for developing abnormal heartbeats also known as premature ventricular contractions, atrial fibrillation and atrial flutter. [3][4] Patients were also observed to have elevated blood pressure (Hypertension). Most importantly, marijuana use has been shown to be an independent risk factor (5-fold risk in the first hour of marijuana use) for heart attacks (myocardial infarction). [5][6]

Cannabis has been shown to affect affect bleeding patterns in patients — which can be a significant consideration during surgery. Several studies have discussed marijuana’s anticoagulation or blood thinning effects.[7][8]Cannabinoids and their metabolites, of which there are over 400, can stop platelets from clumping together, a crucial step in the process of clot formation. Furthermore, this has been shown to be a dose-dependent relationship, implying that high-dose marijuana consumers have a higher risk of bleeding diathesis during surgery.[9]Increased bleeding means increased risk for blood transfusions. Bleeding can also obstruct the surgeon’s field of vision while operating making it harder to complete the surgery.

Conversely the decreasing platelet count caused by cannabis can cause some patients to form clots abnormally. [10]The increased clot formation could be why patients who use cannabis have a higher risk of developing heart attacks and strokes. [11]

Finally cannabis can interact with many drugs used in anesthetic practice. Patients who consume cannabis regularly can have significantly increased anesthetic requirements. A randomized, double-blinded study showed that routine marijuana users required significantly higher doses of propofol (almost twice the normal dose) for appropriate sedation.[12]THC is primarily metabolized in the liver, by the CYP 450 system of enzymes; furthermore, THC and CBD concentrations are high in the liver after oral ingestion, which results in an even greater hepatic load for metabolism [14]. This underpins the reason why patients who consume cannabis may have an increased tolerance for opioids, chlorpromazine and barbiturates, which are routinely used in sedation and are also metabolized by the liver.

Therefore most practitioners recommend that patients hold off on consuming cannabis for 3–4 weeks before their surgery. As states push the legalization of recreational marijuana, more research is being done to study the affects of cannabis on those who are undergoing anesthesia and surgery.

However, CBD an active ingredient in cannabis may offer its own anti-flammatory and analgesic benefits. Recently,Epidolex, the first prescription, plant-derived cannabinoid medicine was approved in the United States as the first in a new class of anti-epileptic medications. This was a groundbreaking approval because it was the first CBD product approved by the FDA. Epidolex is only approved for treatment of seizures associated with Lennox-Gastaut syndrome (LGS) or Dravet syndrome but it’s a significant move by the FDA. Anecdotally patients have seen benefits of CBD with the treatment of ulcerative colitis, gastritis, arthritis and other conditions. There may be benefits for CBD with chronic pain conditions but research needs to be done about the clinical benefits of CBD. If a patient is taking CBD, it is important to make their physicians aware about their use.