Dogs should never drink caffeinated coffee, but if a dog accidentally consumes some, it is unlikely he or she will suffer any ill effects. The most dangerous ingredient for dogs in coffee is the caffeine, which can lead to an increased heart rate and seizures in some dogs.
While caffeinated coffee is certainly not a healthy drink for a dog, most dogs can drink a small amount of coffee without experiencing any side effects. This depends largely on the individual dog's health and weight. The Nest explains that a cup of coffee usually contains less than the amount of caffeine considered lethal even for a small dog. However, there is a much higher concentration of caffeine in coffee grounds, so a dog that is okay after having a sip of coffee might have a more serious reaction if it ingests coffee grounds.
According to the Pet Poison Hotline, signs of caffeine poisoning in canines range from milder symptoms such as restlessness and vomiting to severe symptoms like seizures. Some dogs may collapse after consuming too much caffeine. Because caffeine causes an increased heart rate, dogs with heart conditions are in even more danger when they have access to coffee. If your dog truly enjoys the taste of coffee, decaffeinated coffee is a safer alternative in small portions.