Several common supplements can cause dangerous bleeding, interfere with anesthesia, or destabilize your heart rate during surgery. Most surgeons ask patients to stop all herbal supplements and certain vitamins at least two weeks before a scheduled procedure, though some require a longer window. Here’s what to stop, why it matters, and when.
Supplements That Increase Bleeding Risk
The biggest concern with supplements before surgery is uncontrolled bleeding. Your body relies on platelets clumping together to form clots and stop blood loss at the surgical site. Several popular supplements interfere with that process.
Garlic tops the list. Multiple randomized controlled trials show garlic decreases platelet aggregation, and case reports have repeatedly linked garlic supplementation to surgical bleeding, even in patients not taking blood thinners. A 2022 review in the Proceedings of Baylor University Medical Center classified garlic’s association with surgical bleeding as “strong” and recommended it be discontinued before any procedure.
Hawthorn, an herbal extract sometimes taken for heart health, carries a similar warning. A cohort study found that hawthorn extract increases bleeding risk during cardiac surgery, and the same review classified it alongside garlic as a supplement that should be stopped before an operation.
Vitamin E is another one to flag. At supplemental doses, vitamin E reduces platelet aggregation and competes with vitamin K, the nutrient your body needs to produce clotting factors. Vitamin E toxicity can cause major bleeding events, including intracranial hemorrhage. Drug interactions have been reported in patients taking more than 300 IU daily, and doses above 400 IU were found to be harmful in patients experiencing acute stroke. If you take a standalone vitamin E supplement rather than just a multivitamin, let your surgical team know.
Flaxseed oil and grape seed extract also affect clotting. Randomized trials show flaxseed oil decreases platelet aggregation, while grape seed extract increases the time it takes platelets to seal a wound.
Extra Caution If You Take Blood Thinners
Some supplements pose little bleeding risk on their own but become dangerous when combined with anticoagulant medications like warfarin. A chart review of more than 807,000 patients found that taking ginkgo biloba with warfarin significantly increased the risk of major bleeding events compared to warfarin alone. Chondroitin-glucosamine, a popular joint supplement, showed a similarly convincing association with bleeding in patients on warfarin.
Case reports have also linked warfarin-related bleeding to turmeric, melatonin, chamomile, fenugreek, milk thistle, bilberry, peppermint, and cinnamon. Each of these was connected to elevated INR (a measure of how slowly your blood clots) or outright bleeding episodes in patients on anticoagulants. If you take any blood thinner, even a newer one, bring a full list of your supplements to your pre-surgical appointment.
Supplements That Interfere With Anesthesia
St. John’s wort is one of the most problematic supplements in a surgical setting because it changes how your liver processes drugs. Its active compound, hyperforin, activates a set of liver enzymes that break down many common anesthetic agents faster than expected. That can mean your anesthesia wears off too soon or doesn’t work as intended. One documented case involved a 21-year-old woman who experienced delayed emergence from anesthesia after months of St. John’s wort use. Because it takes time for those liver enzymes to return to normal, the American Society of Anesthesiologists has advised stopping St. John’s wort two to three weeks before surgery, a longer lead time than most other supplements require.
Kava and valerian root, both taken for anxiety or sleep, create the opposite problem. Instead of speeding up drug metabolism, they amplify the sedative effect of anesthesia. Both work through the same brain signaling system (GABA receptors) that general anesthetics target. When anesthetic drugs arrive at receptors already primed by kava or valerian, the combined sedation can be deeper and longer than the anesthesiologist planned. That makes dosing unpredictable and recovery harder to manage. Kava’s effects are dose-dependent, meaning heavier users face greater risk.
Supplements That Affect Heart Rate or Blood Pressure
Ephedra (also labeled ma huang) is an herbal stimulant that can spike blood pressure and cause erratic heart rate during surgery. It was banned from U.S. dietary supplements in 2004 but still appears in some weight-loss products sold online or internationally. If you’ve taken anything containing ephedra, your anesthesia team needs to know, because sudden blood pressure swings mid-surgery can be life-threatening.
High-dose caffeine supplements and pre-workout formulas can create similar cardiovascular instability under anesthesia. If you regularly use concentrated caffeine pills or pre-workout powders, plan to taper off before your procedure rather than stopping abruptly, which can trigger withdrawal headaches that complicate recovery.
Supplements With Conflicting or Unclear Evidence
Not every supplement on a “stop before surgery” list has strong evidence behind it. Fish oil (omega-3 fatty acids) is a good example. For years, patients were told to stop fish oil one to two weeks before surgery due to theoretical bleeding risk. But the clinical data hasn’t consistently supported that concern. Some surgeons still ask you to stop it as a precaution, while others no longer consider it necessary. Follow your own surgeon’s instructions on this one.
Ginger falls in a similar gray area. Four randomized trials found it reduces platelet aggregation, while four others found no effect. Ginseng is another surprise: despite its reputation, controlled trials show it does not increase bleeding, and one study found it actually reduced warfarin’s anticoagulant effect. Saw palmetto, commonly taken for prostate health, also showed no association with bleeding in the evidence reviewed.
The conflicting data doesn’t mean these supplements are safe to keep taking. It means the risk is uncertain, and most surgical teams will still ask you to pause them.
When to Stop and What to Tell Your Surgeon
The general rule most surgeons follow is to stop all herbal and dietary supplements at least two weeks (14 days) before a scheduled surgery. St. John’s wort requires a longer window of two to three weeks because of how persistently it alters liver enzyme activity. Garlic, hawthorn, ginkgo, and vitamin E should also be stopped within that two-week frame to allow platelet function and clotting factors to normalize.
There is no single official guideline that covers every supplement. The American Society of Anesthesiologists does not currently have a formal position statement or practice guideline addressing herbal supplement discontinuation before surgery. That means your surgeon’s office sets its own protocol, and those protocols vary. Some will hand you a printed list. Others rely on you to volunteer the information.
The most important thing you can do is bring a complete, written list of every supplement, vitamin, herbal tea, and over-the-counter product you take to your pre-operative appointment. Include the brand, the dose, and how often you take it. Surgeons and anesthesiologists can’t adjust their plan for risks they don’t know about, and supplements don’t show up on standard blood work. If you forgot to stop a supplement and your surgery is days away, call the surgical office rather than guessing. In some cases the procedure can go forward safely; in others, rescheduling is the smarter choice.