 Humans have been poisoned after ingesting the leaves, a particular problem during World War I when the leaves were mistakenly recommended as a food source in Britain.The toxic rhubarb leaves have been used in flavouring extracts, after the oxalic acid is removed by treatment with precipitated chalk (i.e., calcium carbonate).The (median lethal dose) for pure oxalic acid in rats is about 375 mg/kg body weight, or about 25 grams for a human.