Herb-Drug Interactions: Willow

2011

Salix species (Salicaceae)

Synonym(s) and related species

European willow, Salix, White willow.

Salix alba L., Salix cinerea L., Salix daphnoides, Salix fragilis L., Salix pentandra L., Salix purpurea L.

Pharmacopoeias

Willow Bark (British Ph 2009, European Ph 2008); Willow Bark Dry Extract (British Ph 2009, European Ph, 6th ed., 2008 and Supplements 6.1, 6.2, 6.3 and 6.4).

Constituents

The bark of willow contains the phenolic glycosides salicin (up to 10%), acetylsalicin, salicortin, salireposide, picein, triandrin. Esters of salicylic acid and salicyl alcohol, and flavonoids and tannins are also present. Extracts are sometimes standardised to a minimum of 1.5% of total salicylic derivatives, expressed as salicin (British Ph 2009, European Ph, 6th ed., 2008 and Supplements 6.1, 6.2, 6.3 and 6.4).

Use and indications

The bark of willow is reported to have analgesic, anti-inflammatory, antipyretic and astringent properties. It has long been used for treating all kinds of fevers, headache, influenza, rheumatism, gout and arthritis.

Pharmacokinetics

In a pharmacokinetic study, 10 healthy subjects were given two oral doses of Salix purpurea bark extract, each standardised to contain 120 mg of salicin, 3 hours apart and 30 minutes before meals. Salicylic acid was the most prominent metabolite detected in the serum, with peak levels achieved approximately 1 hour after the dose. The AUC of salicylic acid from the willow bark extract was equivalent to the AUC of salicylic acid from an 87-mg dose of aspirin (given to healthy subjects in another study). However, it is not clear if this amount of salicylic acid has the same antiplatelet effects as aspirin: one study found that taking an extract of the bark of Salix purpurea and Salix daphnoides, to achieve a salicin dose of 240 mg daily, had a much smaller effect on platelet aggregation than aspirin.

Interactions overview

No interactions with willow found. It has been suggested that willow bark is likely to interact with antiplatelet drugs and NSAIDs (which have antiplatelet effects), and increase the risk of bleeding with anticoagulants. This is because one constituent, salicin, is metabolised to salicylic acid, a substance that is also derived from aspirin. Given that pharmacokinetic studies (see above) suggest that doses of willow bark extracts can achieve levels of salicylic acid that are equivalent to an 87-mg dose of aspirin, this seems reasonable. However, other studies suggest that the antiplatelet effects of aspirin are much greater than those of willow bark, which suggests that willow bark extracts may be less likely to interact than aspirin. The antiplatelet effects of willow bark need much more research before any firm recommendations can be made about its potential to interact with antiplatelet drugs, NSAIDs and anticoagulants; however, until more is known some caution is warranted.

The concurrent use of willow bark and antiplatelet drugs (such as aspirin or clopidogrel) need not be avoided: indeed combinations of antiplatelet drugs are often prescribed together, but it may be prudent to be aware of the potential for increased bleeding. Patients should discuss any episode of prolonged bleeding with a healthcare professional.

Clinically, the use of an antiplatelet drug with an anticoagulant should generally be avoided in the absence of a specific indication. It may therefore be prudent to advise against concurrent use with willow bark. However, if concurrent use is felt desirable it would seem sensible to warn patients to be alert for any signs of bruising or bleeding, and report these immediately should they occur.

This advice is probably applicable to any herb with known antiplatelet effects.