Chill Out Brings Doubt


Researchers in Australia* are warning that there “is an urgent need for research” to confirm the efficacy and safety of L-tryptophan supplementation in horses. A naturally occurring amino acid found in human foods such as turkey, chicken, pork and cheese, L-tryptophan is also a main ingredient in several equine calming supplements on the market today. It’s long believed that L-tryptophan has a calming effect in animals, including humans and horses.

However, the warning out of Australia cites that L-tryptophan’s calming effect is “species-dependent, and there are no scientific publications that confirm the efficacy of L-tryptophan as a calmative in excitable horses. The few studies where L-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 haemolytic anaemia [destruction of red blood cells] if given orally.”

In 1989 the Food and Drug Administration issued a nationwide recall, which is still in place today, of all over-the-counter dietary supplements for humans containing 100 milligrams or more of L-tryptophan, and in 1990 it prohibited importation of the substance. The ban was issued after a link was established between L-tryptophan supplementation and Eosinophilia-Myalgia Syndrome, a potentially deadly disease in humans that can lead to congestive heart failure.

When it comes to equestrian sport, the United States Equestrian Federation’s assistant executive director, John G. Lengel, DVM, says that under the USEF’s Drugs and Medications Rule, L-tryptophan is considered a nutrient, not a drug, and therefore is not a forbidden substance. However, he says, “L-tryptophan, when administered for the purpose of calming a horse in competition, violates the spirit of the USEF’s rule.” Many calming supplements that contain L-tryptophan also contain other ingredients that are banned under USEF rules, and in these instances the supplement would not be allowed.

*“Calmatives for the excitable horse: A review of L-tryptophan.” Grimmett A., Sillence M.N., School of Agriculture, Charles Sturt University, Australia



  1. interesting. I have a 14 yr. high strung gelding that I give Calm and Cool in his feed 2x daily. I’m going to re-read the ingredients, but I don’t believe it has tryptophan in it. Will have to research his supplements more throughly in the future. Thank you.

  2. I supplement my high strung gelding with B vitamins and a supplement that has L-tryptophan and I notice a big difference in his demeanor. Without his supplements he won’t pay any attention to me and is a danger to himself. I will continue to give him his supplement for his own safety

  3. Has anyone noticed a link between L-trytophan and colic? My horse has colicked twice now, both times the day after I gave him “So Calm” which contains L-tryptophan….

  4. This is a very interesting article to me as I use a calming supplement when I show my gelding and they almost always contain L-tryptophan. I’m definitely doing more research on this before I give it to him again!

  5. One has to when the author last researched the information about tryptophan. According to the author: “In 1989 the Food and Drug Administration issued a nationwide recall, which is still in place today”…The national ban ended in 2001, with sales of tryptophan commencing again in 2002.
    It’s quite possible the data mentioned by the author has not squared past conclusions with recent ones.


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