Performance and racehorses can often become irritable, nervous and have unpredictable behavior, particularly when placed under stress at competitions and under heavy training regimens. This tends to be worse when they receive high-grain, low roughage diets and when they are confined to stables throughout the day.

Deficiencies of B-group vitamins (B1, B2, B3, B6 & B9), and magnesium have been linked to nervous symptoms and accordingly the feeding of high doses of vitamin B1 (thiamine) has been used to produce a calming effect. Magnesium has also been administered to relieve nervous symptoms in grain-fed horses.