My cat licked a small amount of mustard seed powder from a spice jar — is this toxic?

Yes, mustard seeds and powder are toxic to cats. Even a small amount can cause significant gastrointestinal irritation, leading to vomiting, diarrhea, and abdominal pain. In higher doses, the compounds can potentially affect the heart. Watch your cat closely for any signs of distress or illness. Given the risk, it's best to contact your veterinarian for advice specific to your cat's situation, especially if any symptoms develop.