Although safe when used as a medicine in appropriate doses for dogs and cats, the common St. John's wort can be toxic if consumed in excess. Livestock and horses are particularly vulnerable to its effects, with just a small amount leading to poisoning symptoms. This is due to the hypericin in the flowers and leaves - cattle only need to consume 1% of their body weight, or 4% for sheep, for symptoms to show.In dogs and cats, symptoms begin with photosensitivity, skin ulcers, vomiting, diarrhea, increased heart rate, and convulsions. If the animal continues to consume the plant over a long period of time, they may experience liver damage and skin cancer. Horses and livestock are affected more severely, with symptoms appearing around 5 hours after the plant has been consumed. These include agitation, head rubbing, weakness in the hind limbs, and depression. This then worsens to cause diarrhea, skin inflammation, and an increased body temperature.If you suspect that an animal has been poisoned by the common St. John's wort, remove them from the sunlight immediately, putting them into a dark area. This will prevent photosensitivity from taking its toll. Treatments are available when the poisoning is caught early, but it is vital that you ensure that all pastures are kept free of the plant in the future.