Why does President Snow have blood in his mouth?

In the third book of the series, Mockingjay, Finnick explains that he learned President Snow had a habit of killing his enemies and rivals by poison. So they wouldn't be suspicious, Snow would sometimes takes sips of the poisonous drink himself. He developed bleeding mouth ulcers, and covered the odor by always wearing roses. At the end, he is coughing up blood and apparently very ill from the mouth uclers.