Onions, garlic, leeks, and chives contain thiosulfates (N-propyl disulfide) that oxidatively damage dogs' red blood cells. Heinz bodies form, leading to hemolytic anemia. As little as 15-30 g of onion per kg body weight can be life-threatening.

Initial symptoms (vomiting, diarrhea, abdominal pain) often appear after 1-3 days, not immediately. Blood changes (anemia, dark urine, weakness, rapid breathing) show up after 3-5 days. This delayed effect makes allium poisoning particularly dangerous.

Garlic contains the same thiosulfates but in lower concentration than onions. The toxic dose for garlic is approximately 5 g/kg (fresh). Small, occasional amounts of garlic in dog food or treats are generally considered safe, but regular supplementation is not recommended.