While most people consider chocolate a delicious treat, it can actually be lifethreatening for some animals. This is because chocolate contains two substances that are __1__ many animals: caffeine and theobromine. Both are stimulants from a group of chemicals called methylxanthines. Since they affect the nervous system and the heart, they can cause serious health issues if __2__ in large quantities. Although humans don’t have much difficulty processing methylxanthines, many animals do not have this ability. As a result, methylxanthines can build up in their bodies and reach toxic levels.
The toxicity of chocolate is influenced by several factors, __3__ the type of chocolate, the amount consumed, and the size and species of the animal. Darker chocolate tends to have more methylxanthines than lighter chocolate, making it more dangerous.