Cannabis users needed more anaesthetic, but less than the folklore says

Design

single-centre retrospective cohort, 2008 to 2024, multivariable adjustment

Population

341,199 adults having propofol sedation or volatile general anaesthesia; 7.0% non-medical and 0.9% medical cannabis use

Primary outcome

propofol dose for monitored anaesthesia care; mean end-tidal age-adjusted minimum alveolar concentration for general anaesthesia

Effect

non-medical +6.85 µg/kg/min propofol (95% CI 6.07-7.63) and +0.01 MAC; daily recreational users +8.41 µg/kg/min (7.00-9.81)

The belief that habitual cannabis users need substantially more anaesthetic is widespread and has rested on small studies. This retrospective cohort took 341,199 adults having propofol sedation or volatile-based general anaesthesia at one American centre between 2008 and 2024, separating medical cannabinoid certification (3,193 patients, 0.9%) from non-medical use (23,896, 7.0%).

The direction was confirmed and the magnitude was not. Non-medical cannabis use was associated with a propofol dose 6.85 µg/kg/min higher (95% CI 6.07-7.63) and volatile anaesthetic 0.01 MAC higher (0.01-0.02). Medical cannabinoid use gave +2.87 µg/kg/min propofol (0.88-4.86) and +0.02 MAC (0.01-0.03). The effect was largest in daily recreational users at +8.41 µg/kg/min (7.00-9.81). At comparable induction doses, non-medical users had a slightly greater heart rate rise after intubation, 1.45 beats per minute (1.10-1.79).

The authors' own conclusion is the honest one: these effects are smaller than previously reported and their clinical relevance is unestablished. An extra 0.01 to 0.02 MAC is inside the range a clinician adjusts within continuously; 7 µg/kg/min of propofol on a sedation infusion is real but modest. A 1.45 beat per minute difference in heart rate response is a statistical finding rather than something anyone would notice.

So the practical change is in what to expect rather than in what to give. Asking about cannabis use at the preoperative visit remains worthwhile — it identifies a patient likely to need a little more, and it opens the conversation about frequency, which was where the effect concentrated. It does not justify a protocolised dose increase, and it does not explain a patient who is unexpectedly light.

• Ask about cannabis use and its frequency preoperatively; the effect concentrated in daily users
• Expect modestly higher propofol and volatile requirements rather than a large dose increase
• Titrate to effect as usual — the differences here sit inside normal adjustment ranges
• Do not attribute unexpected intraoperative awareness or lightness to cannabis use on this evidence
• Treat the raised heart rate response after intubation as a statistical finding rather than a clinical one

Why it matters

The teaching that cannabis users need much more anaesthetic has been driving dose decisions on evidence this study shrinks.

Don't overread it

Single-centre retrospective data with record-based exposure cannot establish that cannabis causes the higher requirement, and the effect sizes may not be clinically meaningful.

The statistics, in plain English

With 341,199 patients, very small differences become statistically significant: a 1.45 beat per minute difference in heart rate has a tight confidence interval and a tiny P value while meaning nothing at the bedside. Statistical significance and clinical significance separate completely at this sample size, and the authors say so. Exposure was identified from the record, so misclassification is likely in both directions — undisclosed use would dilute the effect, while the patients recorded as users may differ systematically in ways adjustment cannot capture.

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