Recent research points to tartaric acid as the likely toxic component in grapes and raisins. Dogs cannot properly process it and can develop vomiting and diarrhea within 6-12 hours, progressing to acute kidney failure within 48 hours. The danger is unpredictable — some dogs die from just a few grapes while others eat large amounts with no ill effect. Published case reports in cats are rare, but clinical data suggests a real risk of kidney injury, so grapes and raisins should never be given to cats either.