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).
Side Effects and Warnings
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, an excess of triglycerides in the blood, diarrhea, heartburn, vomiting, and dyspepsia (upset stomach). Willow bark may also lead to 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, blood in the vomit, black stools, abdominal pain, pale mucous membranes, and 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.
Pregnancy and Breastfeeding
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.
Adults (18 years and older)
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.
Children (younger than 18 years)
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.
Interactions with Drugs
Use of acetazolamide and salicylates may cause lethargy, incontinence, and confusion. Caution is advised in patients taking acetazolamide, a carbonic anhydrase inhibitor, which is often used in treating glaucoma or acute mountain sickness. Taking carbonic anhydrase inhibitors with willow bark may increase the therapeutic and toxic effects of both the carbonic anhydrase inhibitor and the salicylate.
The combination of salicin, which is found in willow bark, and alcohol may increase the risk of gastrointestinal bleeding and gastritis.
Salicis cortex extract may increase the risk of bleeding when taken with drugs that increase the risk of bleeding. Some examples include aspirin, anticoagulants ("blood thinners") such as warfarin (Coumadin®) or heparin, anti-platelet drugs such as clopidogrel (Plavix®), and non-steroidal anti-inflammatory drugs (NSAIDS) such as ibuprofen (Motrin®, Advil®) or naproxen (Naprosyn®, Aleve®). Willow bark may also have anti-inflammatory effects. When willow bark is taken in combination with sulfinpyrazone, it may theoretically result in additive anti-platelet effects, which may increase bleeding time.
Willow bark extract may induce excess of triglycerides in the blood or cause blood pressure instability. Patients taking blood pressure medications should consult with a qualified healthcare professional, including a pharmacist. Monitoring may be necessary. Theoretically, the concomitant use of beta-blockers with willow bark may impair the effectiveness of beta-blockers due to willow bark's proposed aspirin-like pharmacological actions.
Theoretically, the concomitant use of diuretics with willow bark may reduce the effectiveness of diuretics and may enhance the risk for salicylic acid toxicity. White willow may also decrease the kidney excretion of methotrexate resulting in toxic levels due to its salicin content.
Theoretically, the concomitant use of phenytoin (Dilantin®) with willow bark's salicylates may increase the Dilantin® levels in the blood, resulting in toxicity.
Theoretically, the concomitant use of probenecid with willow bark may impair the effectiveness of probenecid.
Due to the plasma protein-binding salicylate component of white willow bark, some other plasma protein-bound drugs may be displaced, possibly resulting in altered drug levels. Consult with a qualified healthcare professional, including a pharmacist, for a full list of these agents.
Willow bark plus spironolactone may result in antagonistic or additive effects.
Theoretically, the concomitant use of sulfonylureas with willow bark may increase the effect of sulfonylureas, possibly increasing the side effects and toxicity.
Theoretically, white willow bark may impair the effectiveness of valproic acid.
Interactions with Herbs and Dietary Supplements
Willow bark extract may induce excess of triglycerides in the blood or cause blood pressure instability. Patients taking herbs and supplements that affect blood pressure or cholesterol should use willow bark cautiously.
Willow bark may be contaminated with high levels of cadmium, which may increase concentrations of cadmium in the body. Consumers should select tested brands to avoid using contaminated products.
Theoretically, the concomitant use of diuretics (increases urine flow) herbs or supplements with willow bark may reduce the effectiveness of the diuretic and may enhance the risk for salicylic acid toxicity. Consult with a qualified healthcare professional, including a pharmacist, before combining therapies.
Willow bark may increase the risk of bleeding when taken with herbs and supplements that are believed to increase the risk of bleeding. Multiple cases of bleeding have been reported with the use of Ginkgo biloba, and fewer cases with garlic and saw palmetto. Numerous other agents may theoretically increase the risk of bleeding, although this has not been proven in most cases.
Willow bark may have anti-inflammatory effects and may interact positively with guaiacum resin, black cohosh, sarsaparilla, and poplar bark to reduce chronic arthritic pain symptoms.
The concomitant administration of tannin-containing herbs or supplements may result in the malabsorption of salicylic acid.