"In addition to a high fat content, chocolate contains caffeine and
theobromine, two different types of stimulants that affect the central
nervous system and the heart muscle of dogs, as well as increasing the
frequency of urination.
If
your 50-pound dog gets his paws on a single chocolate-chip cookie, it
probably won't cause him serious problems. However, if he gobbles up
more – a pan of brownies, say – he may develop vomiting or diarrhea.
Once
toxic levels are reached, the stimulants kick in, and this is when you
really have to worry. Symptoms include: restlessness, hyperactivity,
muscle twitching, increased urination and/or excessive panting. If your
pet isn't treated, he could go into a seizure – possibly even die."