Side Effects. White Oak bark is safe for consumption for a very short period of time (say 3 days) for the treatment of diarrhea. Over dose of the herb may cause the following side effects – stomach problems, kidney failure and liver damage. The herb should not be used for more than 2 weeks at a stretch. White Oak tree bark extracts should not be used in the following – pregnant women, lactating mothers, children and those suffering from eczema, hypertonia, fever, kidney and liver problems. Reviews. In the USA, the white oak tree is considered “generally regarded as safe” or GRAS.