In general, dogs will experience onion toxicity if they eat 0.5 percent of their body weight in onions, according to "Veterinary Technician" magazine. So a 50-pound dog will experience onion toxicity after eating 0.25 pounds or 4 ounces of onion. If you know your dog ate onions, contact your nearest emergency vet to begin treatment as soon as possible. For a two-hour window after your pup eats onions, the vet can induce vomiting to expel onions from his system. Afterward, the pet needs veterinary care and red blood cell monitoring. Treatment may require a blood transfusion to help restore your pet's red blood cells to health.