Calmatives for the excitable horse: a review of L-tryptophan

Preparations that contain tryptophan are marketed world wide as calmative agents to treat excitable horses. Tryptophan is the amino acid precursor for serotonin, a neurotransmitter implicated in sedation, inhibition of aggression, fear and stress, in various animal species and humans. Experiments have shown that tryptophan supplementation decreases aggression in humans, dogs, pigs, poultry, and fish, and that it may reduce fearfulness and stress in calves, vixens and poultry. However, behavioural characteristics more closely linked to excitement, such as hyperactivity in dogs, are not modified by tryptophan supplementation. Research using a variety of animals other than horses, has shown that the behavioural response to tryptophan supplementation varies with age, breed and gender, and can be modified by diet, exercise, social status, and level of arousal. Significantly, the response is species-dependent, and there are no scientific publications that confirm the efficacy of tryptophan as a calmative in excitable horses. The few studies where tryptophan has been administered to horses suggest that low doses (relative to those contained in commercial preparations) cause mild excitement, whereas high doses reduce endurance capacity, and cause acute haemolytic anaemia if given orally, due to a toxic hindgut metabolite. As tryptophan continues to be used as an equine calmative, there is an urgent need for research to confirm its efficacy in horses, and to establish a safe therapeutic dose range. In the meantime, available data suggest that it would be imprudent to rely on tryptophan to calm the excitable horse, and instead, that a greater effort should be made to identify the underlying causes of excitability, and to explore more appropriate non-pharmacological remedies.