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It sounds like your dog is vomiting blood. Vomiting blood could be caused by many things including a non-food item in your dog's system, which may have punctured your dog's stomach, Poisoning, ulcers, a blood disorder, and more. It could also be that your dog cut it's mouth and swallowed blood and is now vomiting it up. If an ulcer is to blame, you can give your dog some pepcid or Zantac. Read about Pepcid dosages and usage information here:
Zantac can be given to a dog at .25 to 1mg per pound every 8-12 hours. Read about usage here:
Check your dog's mouth to see if he has a wound in her mouth or a tooth that’s bleeding. If it is not swallowed blood, I urge you to get your dog to the vet.
You can read more about blood in vomit here:
I hope you find this information helpful.