Chewing tobacco and other tobacco products are extremely dangerous for dogs. The nicotine these products contain is toxic and can poison a dog in relatively small amounts.
Most chewing tobacco products contain 6 to 8 milligrams of nicotine per gram; a standard 34-gram tin contains at least 200 milligrams of nicotine. Nicotine toxicity in dogs begins at 1 milligram per pound of body weight, and it can be fatal at 4 milligrams per pound. A 10-pound dog could get sick from just a pinch of tobacco. Consuming a quarter of a can could produce a life-threatening reaction.
Symptoms of nicotine poisoning include tremors, weakness, hyperactivity, drooling, and seizures. Fortunately, nicotine also irritates the gastrointestinal system, so your dog may vomit much of what he eats, reducing his ingested dose. In any case, if you believe your dog has eaten any amount of chewing tobacco, you should call poison control or your vet for immediate assistance.