White willow bark

What it is

White willow bark is the bark of willow trees, taken as a dried extract or a tea for pain and fever. Its marker constituent is salicin, the compound from which aspirin was developed, and extracts are standardised to a stated salicin content such as 120 mg or 240 mg a day. Catechols and flavonoids in the bark are also thought to contribute to its anti-inflammatory activity. Because it is a salicylate source, it carries aspirin-type cautions.

What the research says

For short-term low back pain the evidence is moderate: Cochrane concluded that standardised daily doses of 120 mg or 240 mg of salicin are probably better than placebo for pain and rescue medication use over a few weeks. For arthritis the picture is worse. A 2023 meta-analysis found a small pooled pain benefit of standardised mean difference -0.31 but rated the certainty of that evidence very low, and the two best-designed arthritis trials found no relevant efficacy in osteoarthritis or rheumatoid arthritis. Trials reported no significant adverse events, but published case reports include anaphylaxis, acute respiratory distress syndrome and a fatal liver failure in a toddler, and European regulators contraindicate it under 18 because of Reye's syndrome risk.

Evidence grade: Limited evidence.

What goes wrong

Published case reports of willow bark supplements include anaphylaxis in a woman with aspirin allergy, acute respiratory distress syndrome, and fatal liver failure in a 28-month-old boy who also received acetaminophen. (Source 1)

• Case report, Very low certainty.
• Size: 3 reported cases.
• Who: a 25-year-old woman with acetylsalicylic acid allergy, a 61-year-old woman, and a 28-month-old boy.
• How long: single episodes.
• Result: Anaphylaxis in a 25-year-old woman after a weight-loss supplement containing willow bark; acute respiratory distress syndrome in a 61-year-old woman with sudden dyspnoea and non-productive cough and no prior allergy history; fatal fulminant liver failure in a 28-month-old boy after acetaminophen plus a tea containing willow bark.
• Funding: not stated.

Fatal fulminant liver failure: In a 28-month-old boy after being treated for upper respiratory infection with acetaminophen and a tea containing willow bark.

Salicin affects platelet aggregation, though less than acetylsalicylate, and the clinical significance of that effect is undetermined. (Source 2)

• Expert review, not systematic, Very low certainty.
• Size: not stated.
• Who: willow bark users.
• How long: not applicable.
• Result: Platelet aggregation affected to a lesser extent than acetylsalicylate; clinical significance not established.
• Funding: not stated.

Salicin affects platelet aggregation, although to a lesser extent compared to acetylsalicylate. The clinical significance of this effect on platelet function is yet to be determined.

Position: the EMA herbal committee contraindicates willow bark in people under 18 because of Reye's syndrome risk, and in salicylate or NSAID hypersensitivity, peptic ulcer, G6PD deficiency, coagulation disorders and severe liver or kidney dysfunction. (Source 3)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: European users of licensed willow bark preparations.
• How long: not applicable.
• Result: Formal contraindications in a monograph adopted 31 January 2017.
• Funding: independent (European regulator)

Children and adolescents under 18 years of age due to the risk of Reye's syndrome.

Position: the same EMA monograph advises against taking willow bark together with salicylates or other NSAIDs without medical advice. (Source 4)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: European users of licensed willow bark preparations.
• How long: not applicable.
• Result: Concomitant use with salicylates and other NSAIDs not recommended without medical advice; monograph adopted 31 January 2017.
• Funding: independent (European regulator)

Concomitant use with salicylates and other NSAIDs is not recommended without medical advice.

What the evidence supports

A 2023 meta-analysis of arthritis trials found a small but statistically significant pain benefit over placebo, which the authors graded as very low certainty evidence. (Source 5)

• Meta-analysis, Very low certainty.
• Size: 229 patients with arthritis across five randomised comparisons (review total 329 patients, six RCTs)
• Who: patients with osteoarthritis or rheumatoid arthritis.
• How long: not stated in the pooled result.
• Result: SMD -0.31; 95% CI -0.53 to -0.08; p = 0.007; I2 = 55%; quality of evidence rated very low.
• Funding: not stated.

Our meta-analysis showed a statistically significant difference in pain controls for patients with arthritis between the willow bark and placebo groups (SMD: −0.31; 95% CI: −0.53, −0.08, p = 0.007; I2: 55%; quality of evidence: very low).

A Cochrane review found that standardised willow bark at 120 mg or 240 mg of salicin a day is probably better than placebo for short-term improvement in low-back pain and use of rescue medication. (Source 6)

• Systematic review, Moderate certainty.
• Size: two trials, 261 participants.
• Who: adults with low-back pain.
• How long: short term; the review states its trials tested only short term use, up to six weeks.
• Result: probably better than placebo for short-term improvements in pain and rescue medication, graded moderate quality evidence; a further trial of 228 participants showed relative equivalence to 12.5 mg per day of rofecoxib but was graded very low quality.
• Funding: Cochrane review; it records that authors of eight of the included trials had a potential conflict of interest and four others did not disclose one.

Daily doses of Salix alba (white willow bark), standardized to 120 mg or 240 mg salicin, are probably better than placebo for short-term improvements in pain and rescue medication (two trials, 261 participants, moderate quality evidence).

What the evidence does not support

In a randomised double-blind trial in knee or hip osteoarthritis, willow bark extract at 240 mg salicin a day was no better than placebo, while diclofenac clearly was. (Source 7)

• Randomized trial, Moderate certainty.
• Size: 127 outpatients with hip or knee osteoarthritis (43 willow bark, 43 diclofenac, 41 placebo)
• Who: outpatients with hip or knee osteoarthritis and a WOMAC pain score of at least 30 mm.
• How long: 6 weeks.
• Result: WOMAC pain fell 8 mm (17%) with willow bark versus 5 mm (10%) with placebo; difference -2.8 mm, 95% CI -12.1 to 6.4 mm, p = 0.55; diclofenac versus placebo -18.0 mm, 95% CI -27.2 to -8.8 mm, p = 0.0002.
• Funding: not stated in the abstract.

The difference between willow bark extract and placebo was not statistically significant (–2.8 mm; 95% CI –12.1 to 6.4 mm; p = 0.55, ANCOVA), but the difference between diclofenac and placebo was highly significant (–18.0 mm; 95% CI –27.2 to –8.8 mm; p = 0.0002, ANCOVA).

A parallel randomised trial in active rheumatoid arthritis also found no indication of efficacy for willow bark extract. (Source 7)

• Randomized trial, Low certainty.
• Size: 26 outpatients with active rheumatoid arthritis (13 willow bark, 13 placebo)
• Who: outpatients with active rheumatoid arthritis.
• How long: 6 weeks.
• Result: Pain on a 100 mm VAS fell 8 mm (15%) with willow bark versus 2 mm (4%) with placebo; estimated difference -0.8 mm, 95% CI -20.9 to 19.3 mm, p = 0.93.
• Funding: not stated in the abstract.

The OA study suggested that the willow bark extract showed no relevant efficacy in patients with OA. Similarly, the RA trial did not indicate efficacy of this extract in patients with RA.

Where the evidence is mixed

The same meta-analysis authors said their own findings were not certain enough to rely on because of potential bias. (Source 8)

• Meta-analysis, Very low certainty.
• Size: 329 patients, six RCTs.
• Who: patients with arthritis.
• How long: searches to 12 April 2023.
• Result: Significant pain relief and physical improvement versus placebo, no significant difference in risk of all adverse events, but certainty judged inadequate.
• Funding: not stated.

Owing to the potential bias, the certainty and evidence of our findings are still inadequate. Therefore, further RCTs are needed to confirm our results.

The same Cochrane review recorded that its herbal low-back pain evidence was mainly very low to moderate quality, that only short-term use up to six weeks was tested, and that authors of eight included trials had a potential conflict of interest. (Source 9)

• Systematic review, Low certainty.
• Size: 14 randomised trials across the whole review.
• Who: adults with low-back pain taking herbal medicines.
• How long: up to six weeks.
• Result: evidence mainly very low to moderate quality; a moderate grade was only found for Capsicum frutescens; eight trials had a potential conflict of interest and four other author groups did not disclose conflicts.
• Funding: Cochrane review; conflicts of interest within the included trials are quoted above.

Most included trials were at low risk of bias and the quality of the evidence was mainly very low to moderate. A moderate grade of evidence was only found for C. frutescens. Trials only tested the effects of short term use (up to six weeks). Authors of eight of the included trials had a potential conflict of interest and four other authors did not disclose conflicts of interest.

Within the Cochrane review, one included trial found that willow bark only minimally affected platelet thrombosis compared with a cardioprotective dose of acetylsalicylate. (Source 6)

• Systematic review, Low certainty.
• Size: one trial, 51 participants.
• Who: participants in a trial comparing Salix alba with acetylsalicylate.
• How long: not stated in the review summary.
• Result: minimal effect on platelet thrombosis versus a cardioprotective dose of acetylsalicylate; no numbers are given in the review summary.
• Funding: Cochrane review; funding of this individual trial is not stated in the summary.

S. alba minimally affected platelet thrombosis versus a cardioprotective dose of acetylsalicylate (one trial, 51 participants).

The Cochrane review reported that no significant adverse events were noted within the trials it included, which were small and lasted no more than six weeks. (Source 6)

• Systematic review, Low certainty.
• Size: 14 randomised trials, 2,050 participants in total.
• Who: adults with low-back pain.
• How long: up to six weeks.
• Result: no significant adverse events noted within the included trials; the review does not report adverse event rates.
• Funding: Cochrane review.

Limit of this finding: The review's sentence about adverse events carries no qualifier of its own, but the same review states that its trials tested only short term use, up to six weeks. It is therefore evidence about short, small trials and says nothing about the safety of taking willow bark for longer. An attempt to check this sentence against the Cochrane Library version of the abstract failed, so only the cochrane.org wording could be confirmed and the quote was not extended.

No significant adverse events were noted within the included trials.

Where the research disagrees

Whether willow bark relieves arthritis pain

• Authors of the 2023 meta-analysis in Life, meta-analysis of six randomised comparisons, certainty graded very low: Our meta-analysis showed a statistically significant difference in pain controls for patients with arthritis between the willow bark and placebo groups (SMD: −0.31; 95% CI: −0.53, −0.08, p = 0.007; I2: 55%; quality of evidence: very low). (Source 5)
• Biegert and colleagues, Journal of Rheumatology 2004, two randomised double-blind controlled trials, the osteoarthritis one with an active diclofenac comparator that did separate from placebo: The OA study suggested that the willow bark extract showed no relevant efficacy in patients with OA. Similarly, the RA trial did not indicate efficacy of this extract in patients with RA. (Source 7)

How safe willow bark is

• Cochrane review of herbal medicine for low back pain, systematic review of 14 short-term randomised trials: No significant adverse events were noted within the included trials. (Source 10)
• Memorial Sloan Kettering Cancer Center herb monograph, summarising published case reports, narrative monograph collating individual case reports: Anaphylaxis: In a 25-year-old woman with history of allergy to acetylsalicylic acid, from using a supplement promoted for weight loss that contained willow bark. (Source 1)

How much

• Reference intake: No reference intake exists: willow bark is a plant material used medicinally, not a nutrient. Cochrane describes it by standardised salicin content rather than by any recommended intake. (Source 10)
• Upper limit: No tolerable upper intake level was located. Instead the EMA herbal committee monograph adopted on 31 January 2017 sets contraindications, including use under 18 years because of Reye's syndrome risk, salicylate or NSAID hypersensitivity, active peptic ulcer, G6PD deficiency, coagulation disorders and severe liver or renal dysfunction. (Source 3)
• Studied: Cochrane's included low back pain trials used daily doses standardised to 120 mg or 240 mg of salicin. (Source 10)
• Studied: The osteoarthritis and rheumatoid arthritis trials gave willow bark extract corresponding to 240 mg of salicin per day for six weeks. (Source 7)

A common belief, and what the research shows

The belief: Willow bark is a natural aspirin, so it works like aspirin without aspirin's risks.

What the research shows: Both halves are wrong. On effect, the MSKCC monograph states "Although aspirin is developed from salicin, a direct comparison between aspirin benefits and willow bark benefits cannot be made. At the same time, aspirin-like side effects may occur with willow bark." On risk, the EMA herbal monograph applies aspirin-style contraindications, including "Glucose-6-phosphate dehydrogenase deficiency. Children and adolescents under 18 years of age due to the risk of Reye's syndrome. Severe liver or renal dysfunction. Coagulation disorders." And the two best arthritis trials showed it did not behave like an effective anti-inflammatory: "The OA study suggested that the willow bark extract showed no relevant efficacy in patients with OA."

Questions and answers

What is it?

White willow bark is the bark of willow trees, used as an extract or tea for pain and fever. Its marker compound is salicin, and aspirin was originally developed from that chemistry. It is not, however, interchangeable with aspirin. (Source 11)

What does it do in the body?

Salicin is the compound most of the anti-inflammatory activity is attributed to, with catechols and flavonoids also implicated. Laboratory and animal work describes effects on inflammatory signalling such as TNF-alpha and COX-2, but those are preclinical findings, not demonstrated human outcomes. What has been measured in people is short-term pain relief in low back pain. (Source 12)

Is it good or bad for you?

For short-term low-back pain, a Cochrane review found that standardised willow bark at 120 mg or 240 mg of salicin a day is probably better than placebo for pain and for how much rescue medication people needed, and it graded that evidence moderate quality. That is a narrow claim: the trials were two in number with 261 participants between them, and the review states that only short-term use, up to six weeks, was tested at all. The review also recorded that most of its herbal evidence was very low to moderate quality and that authors of eight included trials had a potential conflict of interest. Nothing in that review speaks to whether longer use is safe or effective. (Source 6)

How do you get more of it?

Willow bark is taken as a standardised extract or as bark tea, not obtained from ordinary food. Trials have used extracts standardised to 120 mg or 240 mg of salicin a day; the osteoarthritis and rheumatoid arthritis trials used 240 mg of salicin daily for six weeks. That is what was studied, not a recommendation. (Source 7)

If it is harmful, what reduces it?

The literature deals with avoidance rather than removal: there is no described method for clearing salicylates from willow bark beyond stopping it. The EMA monograph instead lists who should not take it at all, including anyone under 18 because of Reye's syndrome risk, and people with salicylate or NSAID hypersensitivity, active peptic ulcer, G6PD deficiency or coagulation disorders. (Source 3)

Why might someone be low in it or missing it?

Nobody is low in willow bark. It is a tree bark taken as a medicine, not a nutrient with a requirement, so there is no deficiency state. Its relationship to aspirin also does not make it a substitute for it in either direction. (Source 11)

Which whole foods contain it or feed it?

No whole food supplies willow bark. The plant material itself is the source, taken as a standardised extract or brewed as a bark tea. The tea form is not automatically gentler: one of the published serious cases involved a bark tea given to a small child alongside acetaminophen. (Source 1)

What happens if you do not have it?

Nothing happens from not taking it, since there is no requirement for it. What a person forgoes is a small short-term pain effect: the pooled arthritis estimate was a standardised mean difference of -0.31 against placebo, and the authors rated the certainty of that as very low. (Source 5)

How can you test for it?

No validated test of willow bark status in a person exists in the literature we searched. The closest measurable effect is on platelet function, and the sources say that even this has not been pinned down clinically. Products are instead characterised by their standardised salicin content. (Source 2)

We searched: The 2014 Cochrane review of herbal medicine for low back pain, the 2023 Life meta-analysis of willow bark in arthritis, the 2004 Journal of Rheumatology randomised trials, the EMA herbal monograph on Salicis cortex, and the MSKCC willow bark monograph; none reports a validated test of a person's willow bark or salicin status.

References

1. Memorial Sloan Kettering Cancer Center. Willow Bark - Adverse Reactions, Case reports (Memorial Sloan Kettering Cancer Center, About Herbs). 2021. Read the source
2. Memorial Sloan Kettering Cancer Center. Willow Bark - Adverse Reactions, general reactions and platelet effect (Memorial Sloan Kettering Cancer Center, About Herbs). 2021. Read the source
3. European Medicines Agency, Committee on Herbal Medicinal Products (HMPC). European Union herbal monograph on Salix (various species including S. purpurea L., S. daphnoides Vill., S. fragilis L.), cortex - Contraindications. 2017. Read the source
4. European Medicines Agency, Committee on Herbal Medicinal Products (HMPC). European Union herbal monograph on Salix (various species including S. purpurea L., S. daphnoides Vill., S. fragilis L.), cortex - Special warnings and precautions for use, well-established use column. 2017. Read the source
5. Life (Basel). Willow Bark (Salix spp.) Used for Pain Relief in Arthritis: A Meta-Analysis of Randomized Controlled Trials (pain results). 2023. PMID 37895439, DOI 10.3390/life13102058. Read the source
6. Cochrane Database of Systematic Reviews. Herbal medicine for low-back pain — Cochrane evidence summary, main results. 2014. PMID 25536022, DOI 10.1002/14651858.CD004504.pub4. Read the source
7. The Journal of Rheumatology. Efficacy and safety of willow bark extract in the treatment of osteoarthritis and rheumatoid arthritis: results of 2 randomized double-blind controlled trials. 2004. PMID 15517622. Read the source
8. Life (Basel). Willow Bark (Salix spp.) Used for Pain Relief in Arthritis: A Meta-Analysis of Randomized Controlled Trials. 2023. PMID 37895439, DOI 10.3390/life13102058. Read the source
9. Cochrane Database of Systematic Reviews. Herbal medicine for low-back pain — Cochrane evidence summary, Quality of the evidence. 2014. PMID 25536022, DOI 10.1002/14651858.CD004504.pub4. Read the source
10. Cochrane Database of Systematic Reviews. Herbal medicine for low-back pain (main results). 2014. PMID 24398732, DOI 10.1002/14651858.CD004504.pub4. Read the source
11. Memorial Sloan Kettering Cancer Center. Willow Bark - What is it? (Memorial Sloan Kettering Cancer Center, About Herbs). 2021. Read the source
12. Memorial Sloan Kettering Cancer Center. Willow Bark - Mechanism of Action (Memorial Sloan Kettering Cancer Center, About Herbs). 2021. Read the source