The German Commission E monograph (BGA, Commission E) recommends doses of willow bark extract of 60-120 milligrams of total salicin daily. Clinical studies have used 120-240 milligrams willow bark extract (Assalix®) for four weeks to treat lower back pain. For osteoarthritis pain, 1,360-2,160 milligrams willow bark extract containing 240 milligrams of salicin daily for two weeks has been found effective.
There is no proven safe or effective dose for willow bark in children. Due to the potential for Reye's syndrome from salicylates, children with influenza, varicella (chickenpox), or any suspected viral infection should avoid willow bark.
Avoid in individuals with a known allergy or hypersensitivity to aspirin, willow bark (Salix species), or any of its constituents, including salicylates. Symptoms of allergy may include swollen eyes, pruritus (itching), eczema, anaphylaxis, cough, hoarseness, and dysphonia (abnormal voice).
Willow bark extract has been reported to cause various gastrointestinal problems, headaches, and allergic reactions. Plants containing salicylates have a very bitter taste, so willow bark tea may be unpalatable (unpleasant) for most patients, particularly for children.
Side effects of willow bark may include blood pressure instability, edema (swelling), rash, Hypertriglyceridemia (an excess of triglycerides in the blood), diarrhea, heartburn, vomiting, and dyspepsia (upset stomach). Willow bark may also lead to hyperuricemia (high levels of uric acid in the blood), which may precipitate an attack of gout in susceptible patients. Willow bark may cause hepatic dysfunction, dizziness, fatigue, swollen eyes, bronchospam, papillary necrosis or headaches.
Although not well studied in humans, combination products containing willow may cause acute weakness, hematemesis (blood in the vomit), melena (black stools), abdominal pain, pale mucous membranes, and panhypoproteinemia (low levels of protein in the blood), indicating severe gastrointestinal bleeding.
The salicylates present in willow bark may also impair platelet function resulting in an increased bleeding time. However, daily consumption of salicis cortex extract is thought to affect platelet aggregation to a far lesser extent than acetylsalicylate. Caution is advised in patients with bleeding disorders or taking drugs that may increase the risk of bleeding. Dosing adjustments may be necessary.
Willow bark is not recommended in pregnant or breastfeeding women due to a lack of available scientific evidence. Salicylates are listed as a pregnancy category D; there is positive evidence of human fetal risk with use. Salicylates in breast milk may cause rash in breastfed babies.