N-Propyl Disulfide: The Organosulfur Toxin in Onions and Garlic

N-propyl disulfide is an organosulfur compound that is the principal agent of Allium poisoning in dogs and cats. Alliums (onion, garlic, leeks, chives, shallots) store cysteine-derived sulfur compounds; when the plant tissue is crushed, chewed, cooked, or digested, enzymes and breakdown reactions release reactive sulfur species, including thiosulfinates and thiosulfate-related compounds, that convert to N-propyl disulfide and related disulfides. **Why it harms red cells.** N-propyl disulfide is an oxidizing agent. Inside red blood cells it depletes protective antioxidant capacity and oxidizes hemoglobin, causing the protein to denature and precipitate as Heinz body inclusions. The damaged cells are then removed by the spleen, leading to Heinz Body Anemia in Dogs and Cats. The compound's toxicity is independent of cooking — heating does not destroy it — which is why cooked, powdered, and dehydrated alliums remain dangerous. **Species differences.** The reason this affects dogs and cats (and ruminants like cattle) more than humans relates to differences in red-cell antioxidant defenses and hemoglobin reactivity; cat hemoglobin is particularly susceptible to oxidative damage. Humans tolerate dietary onion and garlic because their red cells are better protected against this specific oxidative pathway.