Most types of mint are safe for dogs. In fact, many dog treats contain mint to freshen the breath and aid with digestion. Mint also contains antioxidants and flavonoids, which give your pet's immune system a boost. However, one type of mint is not safe for dogs. This mint is called English pennyroyal mint. English pennyroyal mint contains pulegone, which is not only toxic to dogs, but people too. Pulegone causes fatal liver damage and impacts other organs when eaten in large amounts. In one study, English pennyroyal was shown to cause massive hepatocellular necrosis in a dog. This caused extreme symptoms, including coughing up blood, bleeding from the nose, and severe diarrhea.