Willow bark: the herbal source of aspirin, evidence for back pain and who must avoid it

The bark of white willow, Salix alba, and related willows contains salicin. The body converts salicin to salicylic acid, which can block enzymes involved in inflammation in a manner similar to aspirin. The chemistry led to the development of aspirin. That kinship explains both the benefits people hope for and the restrictions that apply.

A 2014 Cochrane review of two trials with 261 participants found moderate-quality evidence that daily doses of white willow bark are probably better than placebo for short-term low back pain, with a possible reduction in the use of rescue medication. The wider Cochrane review of herbal medicine for low back pain points out that trials were short and that there is no evidence yet of long-term safety or effectiveness.

A 2019 US Pharmacopeia safety review found no serious adverse events in trials using 120 to 240 mg of salicin daily for up to eight weeks. The European Medicines Agency's herbal monograph recognises short-term treatment of lower back pain with dry extract as a well-established use, and lists traditional uses for minor joint pain, fever in colds and headache.

The EMA monograph sets sensible boundaries. For back or joint pain, use is limited to four weeks. For fever, a person should consult a healthcare provider if symptoms persist beyond three days, and for headache if they persist beyond one day. These limits reflect that persistent symptoms may have a cause that needs diagnosis.

• Back pain that is severe, follows an injury, or comes with leg weakness, numbness around the genitals or bladder or bowel changes needs urgent assessment.

• A fever that lasts or comes with a stiff neck, rash or confusion needs prompt medical help.

• A sudden severe headache needs 999.

The EMA lists contraindications that mirror aspirin: hypersensitivity to salicylates, sensitivity to NSAIDs or asthma triggered by salicylates, active stomach ulcers, severe liver or kidney dysfunction, clotting disorders and glucose-6-phosphate dehydrogenase deficiency. It is also not for use in the third trimester of pregnancy, nor in children and adolescents.

Salicylates carry a risk of Reye's syndrome in children, a rare but serious condition affecting the brain and liver. They can also increase bleeding and may cross the placenta, which is why pregnancy and bleeding disorders are specific concerns.

Reported adverse effects include allergic reactions such as rash, itching and asthma, and gastrointestinal symptoms such as nausea, vomiting, abdominal pain, diarrhoea and heartburn. Their frequency is not defined. Trials in the Cochrane review noted mostly mild, temporary stomach or skin complaints.

Using willow bark together with aspirin or other NSAIDs such as ibuprofen is not recommended without medical advice, because the effects can add together, increasing the chance of stomach irritation and bleeding. The same caution applies for warfarin, DOACs and clopidogrel, and for people with kidney problems or on blood-pressure medicines such as ACE inhibitors, where NSAIDs can cause trouble.

The NHS tells people to check with a pharmacist before taking a herbal medicine if they already take other medicines. A short consultation can prevent a duplication that would never occur with two separate tablet packets, as many people do not think of willow bark as an aspirin-type product.

In the UK, willow bark is sold as registered traditional herbal medicines carrying a THR mark and as unregistered food supplements. The mark means the MHRA has assessed safety, quality and the leaflet, not that effectiveness has been proven. Products without registration may vary in salicin content, so a labelled, registered product is the sounder choice. The NHS advises against buying unlicensed herbal medicines online.