Willow Bark

What it is

White willow bark (Salix alba) is the bark of a willow tree. Its main active compound, salicin, is turned into salicylic acid in the body [1]. Trials used standardised extracts giving 120 to 240 mg of salicin a day [2].

How it might work

Proposed: the pain relief comes from salicin and salicylic acid, which are related to aspirin [1]. Lab studies suggest the whole extract lowers inflammatory signals, and that salicin alone does not explain its effect [3]. These are lab findings, not established in people.

What the research says

For low back pain, a Cochrane review found that white willow bark probably reduces pain and rescue-medicine use more than placebo in the short term, though its evidence is of moderate quality [2]. In a 4-week trial of 210 people with flare-ups of chronic low back pain, 39% of those on the 240 mg dose were pain-free in the final week, against 6% on placebo [4]. In an open trial of 228 people, the extract gave pain relief similar to rofecoxib 12.5 mg a day [5].

For osteoarthritis the trials conflict. A 2-week trial of 78 people found a small pain benefit, with WOMAC pain scores down 14% on extract and up 2% on placebo [6]. A 6-week trial of 127 people found no significant difference from placebo, while diclofenac did better [7]. A 2009 review described the two willow bark osteoarthritis studies as giving disparate results [8]. A 2023 meta-analysis of five studies in 329 people with arthritis found pain relief and better physical function than placebo, but its authors say the certainty of the evidence is still inadequate [9].

A 24-week observational study of 436 people with mostly osteoarthritis or back pain found pain scores fell, but it had no control group [10].

Doses used in studies

Most trials gave 120 or 240 mg of salicin a day [4][11]. Trials lasted 2 to 6 weeks [6][7]. The 2-week osteoarthritis trial used 240 mg a day [6]. The observational study followed people for 24 weeks [10].

Safety and interactions

Common side effects: the most common are stomach-related, and a few allergic reactions were reported [1]. One patient in a 4-week back-pain trial had a severe allergic reaction, perhaps to the extract [4]. The 2023 meta-analysis found no significant difference in adverse events from placebo [9].

Upper limits and toxic doses: no upper limit or toxic dose was found in the studies reviewed.

Medicine interactions: willow bark is listed among herbs that may increase bleeding with warfarin, but this rests on lab, animal and case reports and needs confirmation [12]. Its platelet effect in a small trial was far smaller than that of 100 mg aspirin [13]. In a long-term observational study, NSAIDs and opioids were allowed alongside the extract, and no relevant interactions were reported [10]. No data were found on interactions with diabetes or blood-pressure medicines, antidepressants or immunosuppressants.

Pregnancy and breastfeeding: no trials were done in pregnant women, and the US Pharmacopeia labelling statement says the product is not for pregnant or nursing women [1].

People with conditions: the US Pharmacopeia review notes a risk of increased bleeding in vulnerable people, and says children are at risk of Reye syndrome and that people sensitive to aspirin should avoid it [1]. No liver or kidney data were found in the studies reviewed.

Caveats and open questions

The trials are small and short, and the Cochrane review found poor reporting across them [2]. The Cochrane review had a senior author who previously worked for a company that produced ingredients reviewed in it [2]. Many trials used branded extracts such as Assalix [5] and STW 33-I [10], so results may not carry over to other willow bark products. Combination products such as Instaflex [14] and Pain Bloc-R [15] contain willow bark with other ingredients; one Pain Bloc-R author is an employee of the company LifeSeasons [15]. No ongoing willow bark trials were found in the trial registry for low back pain or osteoarthritis.