21.3 Cannabinoids, Herbal Remedies & Dietary Supplements
Key Takeaways
- The 2023 ASA/ASRA perioperative cannabis guideline recommends screening every patient and delaying elective surgery at least 2 hours after smoked cannabis because of acute tachycardia and airway irritability.
- Chronic heavy cannabis use increases propofol and sedation requirements and increases postoperative pain and opioid consumption rather than reducing it.
- Cannabidiol inhibits CYP3A4 and CYP2C19 and raises midazolam and fentanyl levels, whereas St. John's wort induces CYP3A4 and lowers cyclosporine, warfarin, and oral contraceptive levels.
- Exam-favored herbal discontinuation intervals are ginkgo 36 hours, ephedra and kava 24 hours, garlic and ginseng 7 days, and St. John's wort 5 days, with the ASA recommending 2 to 3 weeks generally.
- Cannabinoid hyperemesis syndrome is cyclic vomiting relieved by hot showers that responds poorly to ondansetron and is treated with topical capsaicin, haloperidol or droperidol, and benzodiazepines.
Why This Topic Matters on the NCE
The NCE content outline lists cannabinoids (I.C.31) and herbal remedies and dietary supplements (I.C.24) as their own pharmacologic classes. Cannabis is now the most commonly used non-prescription psychoactive substance among surgical patients in states where it is legal, and patients frequently do not disclose supplements unless asked directly, because they do not consider them "medications."
1. Cannabinoids
Pharmacology
The cannabis plant contains more than a hundred cannabinoids. Two matter clinically:
- Delta-9-tetrahydrocannabinol (THC) is the psychoactive partial agonist at CB1 receptors (central nervous system, presynaptic terminals) and CB2 receptors (immune tissue).
- Cannabidiol (CBD) is non-intoxicating but is a potent inhibitor of CYP3A4 and CYP2C19, which raises concentrations of midazolam, fentanyl congeners, and warfarin.
Smoked or vaporized THC reaches peak plasma concentration within minutes with effects lasting 2 to 4 hours. Oral (edible) THC is absorbed slowly with a delayed peak at 2 to 4 hours and effects that persist 8 to 12 hours or longer, which is why patients frequently underestimate how recently they are still impaired.
Cardiovascular and airway effects
| Timing of use | Cardiovascular effect | Airway/pulmonary effect |
|---|---|---|
| Acute intoxication | Tachycardia, increased cardiac output, increased myocardial oxygen demand, orthostatic hypotension, conjunctival injection | Bronchodilation acutely, but airway irritation from smoke |
| Heavy chronic use | Bradycardia, blunted sympathetic tone, cannabis-associated myocardial infarction and arrhythmia reported | Chronic bronchitis, airway hyperreactivity, uvular edema, laryngospasm risk |
Anesthetic implications
- Increased anesthetic requirement. Chronic heavy users show cross-tolerance, requiring substantially higher propofol induction doses and higher sedation requirements for endoscopy in observational studies.
- Increased postoperative pain and opioid consumption, not less, despite the analgesic reputation of cannabis.
- Cannabinoid hyperemesis syndrome is cyclic severe vomiting in a chronic heavy user, characteristically relieved by hot showers or baths and poorly responsive to standard antiemetics. It responds to topical capsaicin, haloperidol or droperidol, and benzodiazepines. It is easily mistaken for postoperative nausea and vomiting.
- The 2023 ASA/ASRA joint guideline on perioperative cannabis recommends screening every patient for cannabis use, delaying elective surgery for at least 2 hours after smoked cannabis because of the acute cardiovascular and airway effects, counseling patients about greater postoperative pain and opioid requirement, and avoiding delaying urgent surgery for cannabis use alone.
2. Herbal Remedies and Dietary Supplements
The American Society of Anesthesiologists recommends discontinuing herbal medications 2 to 3 weeks before elective surgery. In practice patients rarely do, so the practical task is recognizing the interaction in front of you.
The bleeding group — the "four G's"
| Supplement | Mechanism | Suggested preoperative discontinuation |
|---|---|---|
| Garlic | Irreversible inhibition of platelet aggregation | 7 days |
| Ginkgo biloba | Inhibits platelet-activating factor | 36 hours |
| Ginseng | Inhibits platelet aggregation; also causes hypoglycemia | 7 days |
| Ginger | Inhibits thromboxane synthetase | 7 days (variable) |
| Vitamin E, fish oil | Platelet inhibition | 7 days |
All of these raise the stakes for neuraxial techniques when combined with each other or with antiplatelet drugs, although current ASRA guidance does not treat herbal supplements alone as a contraindication to neuraxial blockade.
The cardiovascular and sedative group
- Ephedra (ma huang) is a direct and indirect sympathomimetic causing hypertension, tachyarrhythmia, myocardial infarction, and stroke. It depletes catecholamines with chronic use, producing intraoperative hypotension refractory to indirect agents. Discontinue at least 24 hours before surgery. Banned as a dietary supplement in the United States since 2004 but still obtainable.
- Kava potentiates GABA-A and produces marked sedation with anesthetics and benzodiazepines; it is also hepatotoxic. Discontinue at least 24 hours before surgery.
- Valerian is also GABAergic; abrupt cessation can produce a benzodiazepine-like withdrawal syndrome, so it should be tapered rather than stopped, or benzodiazepine cover provided.
- St. John's wort is a potent inducer of CYP3A4 and P-glycoprotein, lowering concentrations of cyclosporine, tacrolimus, warfarin, oral contraceptives, protease inhibitors, midazolam, and alfentanil. It also inhibits serotonin reuptake, creating serotonin syndrome risk with meperidine, tramadol, SSRIs, and methylene blue. Discontinue 5 days before surgery.
- Echinacea is an immunostimulant that has been associated with allergic reactions and hepatotoxicity with prolonged use; it is best avoided in transplant recipients and other immunosuppressed patients.
- Licorice (glycyrrhizin) produces a mineralocorticoid effect: hypertension, hypokalemia, and metabolic alkalosis.
Glucose-lowering supplements
Ginseng, chromium, bitter melon, and fenugreek can cause perioperative hypoglycemia in the fasted patient. Check a glucose on arrival.
Exam Traps
- The exam-favored discontinuation intervals are ginkgo 36 hours, ephedra and kava 24 hours, garlic and ginseng 7 days, and St. John's wort 5 days.
- St. John's wort induces CYP3A4; it does not inhibit it. This is the reverse of CBD, which inhibits CYP3A4.
- Cannabis increases anesthetic and analgesic requirements. Expect to need more propofol, not less.
- Cyclic vomiting relieved by a hot shower in a chronic cannabis user is cannabinoid hyperemesis syndrome and will not respond to ondansetron alone.
A 28-year-old who smokes cannabis daily presents for elective knee arthroscopy and reports last using 30 minutes ago. Applying the 2023 ASA/ASRA perioperative cannabis guideline, what is the most appropriate action?
A transplant recipient maintained on cyclosporine is found to have been taking St. John's wort for six weeks. What is the primary concern?
A chronic daily cannabis user develops severe cyclic vomiting on postoperative day one that does not improve after ondansetron 4 mg and dexamethasone 8 mg. The patient repeatedly asks for a hot shower. What is the most likely diagnosis and appropriate treatment?
Which herbal supplement should be tapered rather than abruptly discontinued before surgery?