The reason why a dog urinates blood the next day after eating fried onions is because both raw and cooked onions contain n-propyl disulfide. After the dog eats it, it will oxidize the hemoglobin, causing acute hemolytic anemia, and then appear Blood in the urine: In addition to blood in the urine, dogs may also suffer from vomiting, diarrhea, poor energy, loss of appetite, etc., which may even be life-threatening, so do not give onions to your dog.