Is anaesthesia less effective on women than men?

Do males and females experience anaesthesia differently? And if so, should their dosages vary accordingly? Many factors influence how anaesthesia affects an individual, including weight, age, and underlying conditions like liver or kidney disease that can alter how drugs are metabolized. Recent research suggests there may be sex-based differences in how general anaesthesia impacts patients.

A 2023 review of 33 studies, highlighted by Live Science, found that women under general anaesthesia were, on average, 38% more likely to experience "anaesthesia awareness" than men. This phenomenon occurs when patients regain some level of consciousness during surgery, ranging from a dreamlike state to full awareness, although they typically do not feel pain. The review also noted that women tended to wake up and respond to questions faster than men.

However, according to Dr. Elisha Peterson, a pediatric pain medicine physician at George Washington University, sex doesn't dictate dosage decisions. Historically, most anaesthesia research has focused on white male subjects, and there isn't enough data on women, particularly those from diverse backgrounds, to tailor healthcare based on sex. Therefore, if a female patient experienced awareness during surgery in the past, her treatment would be the same as a male patient with a similar history: doctors would monitor brain waves and adjust the anesthetic dose accordingly during the procedure.

Research evidence points to differences across genders

Current guidelines for general anaesthesia do not differentiate between men and women. This is partly because research on sex-based differences in anaesthesia sensitivity is mixed. For example, studies on propofol, a commonly used anesthetic, suggested that men tend to wake up more slowly from the drug than women. Further studies have supported the notion that women generally recover faster from anaesthesia, regardless of the anesthetic used.

Dr. Peterson explained that differences in male and female physiology, hormone levels, and metabolic rates may influence how each sex processes anesthetic drugs. Early research in both humans and animals supports this theory. For instance, in 2024, studies found that female mice and women took longer to fall unconscious under anaesthesia but woke up faster than their male counterparts. Male mice that were castrated showed greater resistance to anaesthesia, but when given testosterone, they became more sensitive to its effects.

Emerging research also indicates that anesthetics affect areas of the brain that regulate the sleep-wake cycle, primarily located in the hypothalamus. Since sex hormones like estrogen and progesterone can influence the sleep cycle through the hypothalamus, these hormones may also affect how well anaesthesia works, according to a 2011 study in the British Journal of Anaesthesia.

Additionally, studies in mice have shown sex differences in the dopaminergic system, which regulates wakefulness and attention. Dr. Raveesh Sunkara, a consultant neurosurgeon, suggested this system could play a role in how quickly someone recovers from anaesthesia, though more research is needed to confirm how these brain differences impact sleep and anaesthesia sensitivity.

Despite some research indicating potential sex-based differences, Dr. Peterson emphasized that anaesthesia practices do not currently differ by sex. The overall rate of anaesthesia awareness is estimated to be around 1 to 2 cases per 1,000 procedures, according to the American Society of Anesthesiologists (ASA).

Areas for improvement

Until further studies provide definitive answers, Dr. Peterson suggests some steps could be taken to reduce the incidence of anaesthesia awareness in women. For example, using brain-monitoring tools more frequently during high-risk procedures, such as emergency C-sections, could help.

Additionally, Dr. Kai Rabenstein, an anaesthesia adviser at SEDSConnective, recommends considering a patient’s alcohol consumption. Since alcohol is metabolized similarly to propofol, individuals with high alcohol tolerance may also be more resistant to the effects of anaesthesia. The ASA also advises patients to disclose their past drug and alcohol use, as it can increase the risk of anaesthesia awareness.

By Nazrin Sadigova