Two things makes chocolate dangerous to dogs.
The first is the worst, a chemical called theobromine. The second is rather high levels of caffeine.
A small amount of chocolate is unlikely to cause much harm, but in large amounts theobromine can be very dangerous.
Eating an entire box of chocolates is more dangerous to a dog than to a human because of the difference in their body weights. In addition, the combination of the two stimulants can be worse than either one alone.
Chocolate can cause diarrhea, abdominal pain, muscle tremors, fever and lack of coordination.
Brain effects can range from depression of alertness to seizures and convulsions to coma and death.
If you see your dog wolf down some chocolate, immediately take it to the vet.
There are no antidotes, but quick treatment may help. It involves getting the dog to vomit as much as possible or possibly using a stomach pump.
Medication is given to stop the poison from being absorbed, and supportive therapy, like anti-convulsants, treatment for heart arrhythmia and intravenous fluid to wash the chemicals out.
Other common substances can also hurt pets. Aspirin is not as bad for dogs as for cats, but should still be given only under the advice of a vet, because of side effects like serious stomach ulcers.
As for cats, they have a double problem with aspirin. It is toxic to them, and they cannot metabolize it, so it stays in the body. One dose may last several days.
And do not give your cat Tylenol instead.
Acetaminophen is thousands of times worse. One dose can kill a cat, though dogs tolerate it pretty well.