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How to treat a dog sneezing blood
Keep your dog calm.
If your dog is actively bleeding it may be upset or agitated by that. Focus on keeping the dog calm by petting it and reassuring it if you can.
This will help its mental state, as well as keeping its blood pressure down and thus limiting the amount of bleeding. However, do not give it any medication to keep it calm unless you discuss that option with a veterinarian before you do.