Using Tryptophan to Calm Horses Questioned

In their review of tryptophan research, Australian scientists find that it's too early to conclude tryptophan is effective and safe for calming horses. There's sparse scientific data collected on the effects of tryptophan supplements in horses. Meanwhile, research results on giving tryptophan to various other domestic animals can't be extrapolated to horses.

Tryptophan is an amino acid that the body uses to build serotonin. The neurotransmitter inhibits aggression, fear and stress in animals, including humans. Research has demonstrated that supplements of tryptophan do promote less aggression in people, dogs, pigs, poultry and even fish. It may also enable calves and foxes to feel less fearful and stressed.

But these results can't be applied to horses, the scientists at Charles Sturt University maintain. An animal's response to taking a tryptophan supplement is affected by a myriad of variables including their species, age, breed, gender, diet, exercise, excitement and social status.

The little research conducted up to 2005 on giving tryptophan to horses finds that at doses lower than those contained in commercial tryptophan supplements, horses actually become mildly excited.

High doses of tryptophan also produce undesirable effects. They reduce a horse's endurance capacity. Furthermore, high doses can be toxic if given orally, causing acute haemolytic anaemia where a metabolite in the hindgut causes red blood cells to be destroyed.

Until experiments establish a safe dose for horses, the scientists advise using non-chemical means to calm excited horses.


A. Grimmett and M.N. Sillence. 2005. Calmatives for the excitable horse: A review of L-tryptophan. The Veterinary Journal. 170(1): 24-32.

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