One of the nine essential amino acids that must be supplied in adequate amounts by the diet. It is a precursor for the neurotransmitter Serotonin which is involved in the regulation of mood including anxiety, and sleep. It is touted as a calmative for excitable horses but whilst evidence in other species is supportive no definitive evidence exists in horses, and low doses may actually cause mild excitement. High doses of Tryptophan are potentially toxic and can cause acute haemolytic anaemia due to a toxic breakdown product produced in the hind gut. Until a safe therapeutic dose range has been established use of Tryptophan as a calmative cannot be recommended.