Toxic dose: >5 g/kg body weight (about 0.5% of body weight)
If your dog ate onion, contact your vet — toxicity can be delayed 1-5 days.
All Allium species (onions, garlic, leeks, chives, shallots) are toxic to dogs. They contain thiosulfates that damage red blood cells, causing Heinz body hemolytic anemia. Toxic dose: >5 g/kg for onions (a medium onion weighs ~150 g). Garlic is 3-5× more concentrated. Symptoms may be delayed 1-5 days: weakness, pale gums, rapid breathing, dark urine, vomiting. All forms are toxic: raw, cooked, powdered, dehydrated, and in food items (onion rings, soups, baby food). Treatment: induce vomiting if recent ingestion, activated charcoal, IV fluids, blood transfusion in severe cases. Japanese breeds (Akita, Shiba Inu) are more susceptible.