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To date, there’s not enough research to identify all possible side effects of oak bark tea, supplements, and lotions.
Oak bark is generally considered safe when taken for short periods, specifically 3–4 days for the treatment of acute diarrhea and 2–3 weeks when applied directly to the skin.
Personal accounts suggest that oral forms of oak bark may cause stomach upset and diarrhea. Meanwhile, topical oak bark applications may lead to skin irritation or worsen conditions like eczema, especially when used on broken or damaged skin.
Additionally, high doses and/or the long-term use of oak bark may worsen kidney and liver function.
One study in rats found that doses of 15 mg of oak bark extract per pound (33 mg per kg) of body weight led to kidney damage. *
