Record numbers of dogs are being poisoned after being fed chocolate, grapes and raisins by their owners as treats, say vets. Vets have seen a surge of almost 50 per cent in the number of dogs falling ill after eating chocolate. Chocolate contains theobromine, a naturally occurring stimulant found in the cocoa bean, which affects the central nervous system and heart and can kill a dog. The danger depends on the dog's size and the type of chocolate. Darker and more expensive chocolate contains more theobromine. Humans, and cats, have the metabolism to cope with theobromine. Toxins in grapes, raisins or sultanas can cause renal failure in dogs. In some cases, small dogs have died after eating as few as four grapes. Vets believe many owners are unaware of the dangers and have fed their pets cake and biscuits containing chocolate or grapes as treats. In other cases the animals have helped themselves. There has also been a rise in the number of dogs eating painkillers such as ibuprofen, paracetamol and aspirin. These can sometimes be fatal.Vets believe the animals sometimes find the drugs lying around the house. In many cases, however, pets are given the pills by their owners when they are injured, as it is cheaper than going to a vet.