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Hello, this is Dr. Jess. I am very sorry to hear about your kitty. Vomiting blood (hematemesis) is a very concerning symptom, and not something I would recommend trying to treat at home. A veterinary exam is needed ASAP for him, especially since it sounds like he is lethargic as well.
Possible causes for the problem could include a bleeding stomach ulcer, ingesting a foreign object that has irritated the stomach lining, a problem with blood clotting, a severe case of gastroenteritis, or in older pets we do also worry about cancer of the stomach causing these symptoms.
As I said, there is not an "at home" treatment for this issue. I would strongly recommend a visit to your vet for an exam and they can determine what treatment is needed based on her physical exam.
Here is an article on this issue, FYI: http://www.petplace.com/article/cats/diseases-conditions-of-cats/symptoms/hematemesis-vomiting-blood-in-cats
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