Chocolate may be a factor for heartburn in some people because one of its constituents, theobromine, may affect the esophageal sphincter muscle in a way that permits stomach acids to enter the esophagus. Theobromine poisoning is an overdosage reaction to the bitter alkaloid, which happens more frequently in domestic animals than humans. However, daily intake of 50–100 g cocoa (0.8–1.5 g theobromine) by humans has been associated with sweating, trembling and severe headache. Chocolate contains alkaloids such as theobromine and phenethylamine, which have physiological effects in humans, but the presence of theobromine renders it toxic to some animals, including dogs and cats.