Cassava is classified as sweet or bitter. Farmers often prefer the bitter varieties because they deter pests, animals, and thieves. Like other roots and tubers, both bitter and sweet varieties of cassava contain antinutritional factors and toxins. It must be properly prepared before consumption. Improper preparation of cassava can leave enough residualcyanide to cause acute cyanide intoxication and goiters, and may even cause ataxia or partial paralysis. The more toxic varieties of cassava are a fall back resource (a food security crop ) in times of famine in some places.