During surgery, an anesthesia professional watches oxygen saturation, breathing, heart rate, blood pressure, and other vital signs. So if a patient smokes cannabis, why does the anesthesiologist need to know?
The answer appears to have less to do with oxygen levels and more to do with giving the anesthesiologist information that the monitors cannot provide.
The Finger Monitor Does Not Know You Use Cannabis
The familiar red light clip attached to a finger is a pulse oximeter. It estimates the percentage of hemoglobin carrying oxygen in the blood, called SpO₂.
Another monitor, capnography, measures carbon dioxide in exhaled breath and helps detect whether a patient is breathing adequately.
These are extremely important safety tools, but neither identifies cannabis use.
In fact, research has shown that supplemental oxygen can keep oxygen saturation looking normal even while a patient is hypoventilating. In one controlled surgical study, deliberate reductions in ventilation caused falling oxygen saturation in patients breathing room air, but not in patients receiving supplemental oxygen. Carbon dioxide monitoring provided information about ventilation that pulse oximetry could miss.
More importantly, neither oxygen saturation nor carbon dioxide tells the anesthesiologist how much anesthetic a particular patient may require.
What is the little red oxygen monitor on your finger?
The device is called a pulse oximeter, often shortened to pulse ox. It usually clips onto a fingertip and gives anesthesia personnel two very useful measurements of SpO₂, estimated percentage of hemoglobin carrying oxygen. It also measures how many times the heart is beating per minute.
The device does not actually draw blood. Instead, it shines two wavelengths of light through your finger at approximately 660 nanometers and infrared light around 940 nanometers. Oxygenated and deoxygenated hemoglobin absorb those wavelengths differently. The detector on the opposite side of the finger measures the transmitted light, and the machine calculates an estimate of arterial oxygen saturation.
That red light you can see inside the clip is therefore part of the measurement system, not simply an indicator light. For most healthy people, the U.S. Food and Drug Administration says that a person’s SpO₂ around 95 to 100% is common. Although normal targets can differ depending on lung disease, altitude and the clinical situation.
Why is it important during anesthesia?
Are They Just Being Nosy?
Anesthetic drugs and opioids can slow breathing. An obstructed airway, bronchospasm, aspiration, problems with a breathing tube, or other respiratory complications can also reduce the amount of oxygen reaching the blood. The pulse oximeter provides continuous warning if the patient’s oxygen saturation begins falling.
For a patient who uses cannabis, particularly smoked or inhaled cannabis, this is relevant because airway irritation and bronchial reactivity are among the respiratory concerns anesthesia researchers discuss. But the pulse oximeter is not a cannabis detector and cannabis use does not automatically produce an abnormal oxygen reading. There is another very important limitation, a pulse oximeter measures oxygenation. It does not directly measure whether someone is breathing adequately.
A sedated person receiving supplemental oxygen can sometimes maintain a good looking oxygen saturation for a period even while breathing too slowly and accumulating carbon dioxide. That is why anesthesia professionals commonly use capnography as well. Capnography measures exhaled carbon dioxide and gives the anesthesiologist information about whether air is actually moving in and out of the lungs. Modern anesthesia monitoring therefore combines pulse oximetry with other monitoring rather than relying on the finger probe alone.
Pulse oximeters also have limitations. The FDA warns that accuracy can be affected by poor circulation, motion, cold skin, nail polish, skin pigmentation and other factors. The FDA has specifically been studying differences in pulse oximeter accuracy across skin tones and proposed updated performance recommendations in 2025. Specifically studying “skin tones” does not include the use of cannabis.
Cannabis May Change Anesthetic Requirements
“May” is the Operative Word
That is currently one of the strongest scientific reasons cannabis history may be useful.
A 2026 British Journal of Anaesthesia study examined 341,199 adults receiving propofol sedation or general anesthesia at Beth Israel Deaconess Medical Center.
Researchers found that both medical and nonmedical cannabis users received somewhat higher doses of propofol and inhaled anesthetics. The largest differences occurred among frequent recreational users. The researchers emphasized, however, that the overall increases were modest.
A 2025 systematic review and meta analysis reached a similar conclusion. Across eight studies involving 2,268 patients, cannabis users received an average of about 47mg more propofol than nonusers. However, the evidence varied considerably between studies, so this cannot be turned into a rule saying every cannabis user needs a particular additional dose.
The monitors can tell the anesthesiologist whether you are responding to anesthesia. They cannot tell them beforehand that regular cannabis exposure may be one reason your anesthetic requirement differs from expectations.
Oxygen Can Be Perfect While Something Else Changes
Cannabis can also affect physiology that has nothing to do with oxygen saturation.
After recent cannabis smoking, heart rate and blood pressure can increase. The American Society of Regional Anesthesia and Pain Medicine concluded that these cardiovascular effects are most prominent during the first one to two hours after smoking. Cannabis smoke can also irritate the airway and, in susceptible patients, contribute to bronchoconstriction or chronic bronchitis like changes.
A patient could therefore have an oxygen saturation of 99% while experiencing an altered heart rate or blood pressure response. A pulse oximeter would not explain that.
So What Does Telling Them Actually Add?
Think of the monitors as real time information, your oxygen is 98% while your carbon dioxide is rising. Your heart rate is 105 while your blood pressure is falling. Your cannabis history provides context that you smoked two hours ago, maybe you smoke several times every day, you use a high THC product, or you have used cannabis daily for ten years.
That information may affect how an anesthesia professional interprets changes, anticipates anesthetic requirements, evaluates airway concerns, and plans postoperative pain treatment.
Importantly, the scientific evidence does not establish that every cannabis user faces additional surgical danger. Nor has any of the research cited shown that a person who occasionally used cannabis weeks earlier necessarily has a materially different anesthesia risk.
The evidence is strongest for frequent, chronic, or very recent cannabis use, particularly smoked cannabis. So the scientific reason to tell the anesthesiologist is not simply that they need to watch your oxygen. It is that cannabis use may provide clinically useful information about how your body responds to anesthesia information the monitors themselves cannot provide.
To reference one of the studies from 2025 above saying, “the evidence varied considerably between studies, so this cannot be turned into a rule saying every cannabis user needs a particular additional dose.” If a doctor went into a procedure with the idea that every cannabis user needs 47mg more propofol, then their bias could hurt the patient versus help the patient.
Although there are studies that try to persuade you to tell your anesthesiologist about smoking cannabis beforehand, their fine line details say otherwise. If you tell your anesthesiologist about your cannabis habits before your surgery, they might go into the procedure with a bias. A doctor should never go into a procedure with a bias, they should go off of the monitors in front of them.
With all of that being said, to begin your own home grow, you can get your starting supplies here! We encourage a home grow, so that you can keep your personal eye on what you want to put into your body.
