Another alkaloid found in chocolate is theobromine. Like caffeine, theobromine is a nervous-system stimulant, though much milder. In high enough doses it can be mood-altering, but then there are those words again: in high enough doses. Most chocolates, especially milk chocolates, don’t contain anywhere near enough to create an effect, particularly since theobromine also acts as a diuretic in humans, and gets flushed out of the system fast.
Yet all creatures aren’t created equal when it comes to metabolizing theobromine. Dogs, for example, have a difficult time getting rid of it, which is why chocolate is considered to be dangerous for them. Horses also fall into this category, though being much bigger, they’re not usually in danger of dying from it. The most theobromine usually does in a horse is make it run faster, which is why chocolate is a banned substance at places like Churchill Downs. A trace of theobromine in a urine sample (yes, they test horses for illicit substances too, not just baseball players) is enough to disqualify a horse from a race.