Willow Bark

Scientific Name(S): Salix alba L., Salix purpurea L., Salix fragilis L., and other species. Family: Salicaceae (willow family)

Common Name(S): Willow, weidenrinde, white willow (S. alba), purple osier willow/basket willow (S. purpurea), crack willow (S. fragilis)

Willow Bark is also known by the names White Willow, Black Willow, and Crack Willow. White willow bark is a tree native to Europe and Asia. The name "white willow" comes from the color of the leaves, which are covered with fine white hairs

Botany

Willows are small trees or shrubs, many of which grow in moist places or along riverbanks in temperate and cold climates. Most of the several hundred species are dioecious, with male and female catkins (flowers) on separate plants. Largely insect pollinated, different species of willow hybridize freely. Medicinal willow bark is collected in the early spring from young branches (2 to 3 years of age) of the species listed above. Other species of Salix have similar chemistry and pharmacology.

Uses

Willow bark can be an effective analgesic if the content of salicylates is adequate. White willow bark is used for conditions that cause pain, inflammation, or fever, such as:

• Acute back pain
  
• Fever
  
• Flu
  
• Joint pain

Side Effects

Adverse effects are those of salicylates in general. Use with caution in patients with peptic ulcers and other medical conditions in which aspirin is contraindicated.

Interactions and Depletions

Because willow bark contains salicylates, it has the potential to interact with blood-thinning medications, such as aspirin and warfarin. People who are taking anticoagulant or antiplatelet medications should avoid the use of willow bark althogether, except under strict medical supervision.

History

For centuries, the bark of European willows has been used to treat fevers, headache and other pain, and arthritis. North American willows have also been used in folk medicine. Most of the European medicinal willows have been introduced to the Americas and have escaped cultivation. In the late 19th century, salicylic acid was widely used in place of willow bark, and its derivative aspirin was discovered to be less irritating to the mouth and stomach.

Chemistry

Salicylate derivatives are the primary medicinal constituents of willow bark. While small amounts of salicylic acid can be detected in most species, the principle salicylates of S. alba are the phenolic ester glycoside salicortin and glycoside salicin, its acid hydrolysis product. Salicin is hydrolyzed in the intestine to saligenin (o-hydroxybenzyl alcohol), which is absorbed and then cxidized to salicylic acid. Salicortin and other related salicylates are chemically unstable (for example, to the boiling water in teas) and avoidance of loss of these compounds requires careful drying of the bark. Extraction protocols that avoid decomposition of the native glycosides have been developed. Most standards for medicinal willow bark require salicylates to be greater than 1 % of dry weight; however, this standard is difficult to achieve with many source species. This has stimulated surveys of the salicylate content of many other species of Salix as well as aspen (Populus), which also contains salicylates. While the leaves generally contain lower concentrations of salicylates than the bark, several species contain medicinally useful quantities of salicylates in their leaves. Salicylates have been quantified in willows by spectrophotometry by thin-layer chromatography (TLC), by high-performance liquid chromatography (HPLC) after enzymatic deglycosylation, and by capillary electrophoresis. A method using gas chromatography of silyl derivatives of salicylates gave comparable results to HPLC. An HPLC method was used to compare the salicylate content of different cultivated clones of Salix myrsinifolia grown in a single location. NMR spectra of the principle salicylates of willows have been reported and assigned. The ecological role of salicylates has also been investigated. Naringenin glycosides, oligomeric procyanidins, and condensed tannins presumably derived from the simpler flavonols have been obtained from commercial willow barks.

Summary

Willow bark was approved by the German Commission E for diseases accompanied by fever, rheumatic ailments, and headaches. It is monographed by ESCOP, the British Herbal Pharmacopeia, and is official in the German Pharmacopeia. An American Herbal Pharmacopeia monograph is due to be published shortly.

Willow bark can be an effective analgesic if the content of salicylates is adequate. Adverse effects are those of salicylates in general. Use with caution in patients with peptic ulcers and other medical conditions in which aspirin is contraindicated.