White Willow Bark

📜 Historical Use

Willow bark is Salix alba, known pharmacognostically as Salicis cortex and sold under the common names white willow, black willow and bay willow. MSKCC describes willow as a deciduous tree native to Europe, Asia and some parts of North America, whose bark has been used for thousands of years in China and Europe as a remedy for fevers and pain. That makes willow one of the few remedies catalogued on this site whose traditional indications and modern marketing actually line up: fever and pain then, fever and pain now, with arthritis, headaches, inflammation, influenza and weight loss added to the current list of purported uses. The continuity is real, and it produced one of the most widely used drugs in the world, since aspirin is derived from salicin, the compound in the bark. But continuity of use is not validation of use. The two indications tradition emphasised most, fever and headache, are precisely the two for which MSKCC reports no clinical trial evidence at all, while the indication with the best trial support, low back pain, is not what the historical record foregrounds. And the existence of aspirin is a reason for caution rather than reassurance here: MSKCC warns that a direct comparison between aspirin benefits and willow bark benefits cannot be made, that willow bark should not be used as a substitute for aspirin because of the variation in its potency, and that aspirin-like side effects may occur with willow bark all the same.

📊 What the Research Shows

This page is filed under headache and general pain, and headache is exactly where the evidence is absent. MSKCC lists headaches among willow bark's purported uses and benefits, but offers no efficacy finding for it anywhere in the monograph, not a trial, not a review, not even a mixed result. The same is true of the use the plant is most famous for: on reducing fever, MSKCC says willow bark is thought to act in a similar way as aspirin, but clinical trials have not been performed. The one indication with positive human data is low back pain. MSKCC states that clinical trials suggest willow bark extract is effective in treating low back pain, and in its clinical summary that a few studies suggest willow bark may help manage back pain. Move one joint over and the picture degrades: results for osteoarthritis pain are described as mixed, and for rheumatoid arthritis MSKCC records negative findings outright. Weight loss, which is how willow bark is often sold inside combination products, has nothing behind it at all: MSKCC says there are no studies to indicate that willow bark can aid in weight loss, and notes that a case of allergic reaction was reported following use of a weight-loss product that contained it. The honest summary is therefore a narrow one. Someone with low back pain has some trial evidence to point to. Someone with a headache, a fever, rheumatoid arthritis or a weight-loss goal does not.

Take 240mg standardized extract (containing 60mg salicin) up to 3x daily with food.

Avoid if allergic to aspirin. Not for children under 18 (Reye's syndrome risk).

⚠️ What happens if taken in excess:

Contains salicin, which converts to salicylic acid (aspirin). Excessive doses can cause the same toxicity as aspirin overdose: tinnitus, nausea, vomiting, and in severe cases, metabolic acidosis, seizures, and organ failure. Never give to children under 18 (Reye's syndrome risk). Can cause GI bleeding at high doses.

Two drug interactions are named, and both follow from treating this as a salicylate rather than as a herb. MSKCC's herb-drug interaction entry names warfarin or other anticoagulants: used concurrently, willow bark may increase the risk of bleeding. Its patient-facing do-not-take list repeats warfarin and other blood thinners and adds a second class, the non-steroidal anti-inflammatory drugs, where MSKCC says willow bark may increase the risk of stomach and intestinal damage and bleeding. That NSAID warning is the one most likely to be missed in practice, because ibuprofen and naproxen are bought without a prescription and a herbal pain product is easy to take on top of them. Timing matters too: because of the potential for increased bleeding risk, MSKCC says willow bark should be discontinued before surgery or chemotherapy. It gives no number of days, so how far in advance to stop is a question for the surgical or oncology team rather than for this page. One absence is worth stating plainly, because other sites fill it in. MSKCC names no cytochrome P450 enzyme, no drug transporter, and no CYP3A4 or CYP2C9 effect for willow bark. The interactions it documents are pharmacodynamic, bleeding risk added to bleeding risk and gastric irritation added to gastric irritation, not metabolic. And although the fatal paediatric case above involved acetaminophen, MSKCC does not list acetaminophen as a herb-drug interaction and it should not be presented as one.

1. Willow Bark: Purported Benefits, Side Effects & More — Memorial Sloan Kettering Cancer Center, Integrative Medicine Service MSKCC About Herbs, Botanicals & Other Products (2021)[mskcc ↗]