Salix alba L., S. nigra L., S. × fragilis L., S. purpurea L. and other Salix spp.

Salicaceae

S. alba: European willow, white willow; S. nigra: black willow, pussy willow; S. × fragilis: crack willow; S. purpurea: purple osier

Salicis cortex

Bark

The main constituents are the phenolic glycosides salicin, acetylsalicin, salicortin, salireposide, picein, triandrin. Esters of salicylic acid, salicyl alcohol, flavonoids including ampelopsin, taxifolin and derivatives, catechin and tannins are also present (Agnolet et al. 2012; EMA 2009; Williamson et al. 2013).

Several studies have shown the effectiveness of standardised willow bark extracts in arthritis and related inflammatory conditions. A two week, double-blind, randomised controlled trial assessing the effect of an extract (240 mg salicin/day) in patients with osteoarthritis showed a moderate analgesic effect compared to placebo (Schmid et al. 2001). Another trial demonstrated that the extract (standardised to daily dose of 240 mg salicin) reduced pain to the same degree as 12.5 mg/day rofecoxib (now withdrawn from sale due to cardiovascular toxicity) (Gagnier et al. 2006). A cohort study suggested that willow bark extract (120–240 mg salicin) had a comparable effect to standard therapies (e.g. diclofenac and ibuprofen) with better tolerability and could be used for the treatment of osteoarthritis of the hip and knee (Beer and Wegener 2008).

A recent observational study over 6 months evaluated the dosage and safety of willow bark extract STW 33-I, (Proaktiv®), during long-term treatment with both mono- and combination therapy, in 436 patients with osteoarthritis and back pain. Co-medication with other NSAIDs and opioids was allowed. The results showed that the treatment was effective and well tolerated and the authors suggested that STW 33-I can be used in the long-term therapy of painful musculoskeletal disorders and can be combined with NSAIDs and opioids if necessary (Uehleke et al. 2013).