Extract from the Telegraph:
"The PDSA, Britain's leading veterinary charity, analysed the theobromine content of chocolates - the chemical component poisonous to dogs - and found that luxurious brands of plain chocolate were the most lethal.
A 30g bar of dark chocolate can contain more than 700mg of theobromine, enough to kill a small dog such as a cocker spaniel.
White chocolate contains the least amount of theobromine - about 0.27mg per 30g bar - but the PDSA warns that any chocolate intended for humans can be a risk for pets.
Elaine Pendlebury, its senior veterinary surgeon, said: "I have lost count of the number of chocolate poisoning cases I have seen during my years as a vet. There really is no place in a pet's diet for chocolate."
Toxicity symptoms include vomiting, hyperactivity, salivation and increased heart rate. In severe cases it can cause fits."