Most chocolate contains a stimulant called theobromine. This can affect the central nervous system in canines as it is metabolised by dogs at a far slower rate than you or I. This can result in toxic levels in a dogs body from relatively small amounts that put the dogs heart under a lot of strain. In more serious cases, seizures, coma or death can occur.
The severity of the situation depends on your dogs size and the amount of theobromine eaten. When the amount is higher than 20mg/kg BW symptoms of chocolate poisoning will develop. Symptoms will worsen up until about 100mg/kg BW, which is considered a lethal dose.
It is important to act quickly if you suspect your dog has consumed chocolate, particularly dark/raw chocolate. The more common early signs of chocolate poisoning will be vomiting and diarrhoea. It can take up to 12 hours for symptoms to become show.