Acorns are poisonous to dogs. They contain tannins, which are natural toxins that can irritate a dog's stomach and cause digestive upset such as vomiting, diarrhea, and abdominal pain. In larger amounts, tannins can lead to severe liver or kidney damage. Additionally, acorns can develop mold that produces toxic mycotoxins, posing further health risks. The hard shell of acorns also presents a choking hazard and can cause intestinal blockages. If a dog eats acorns and shows symptoms like vomiting, loss of appetite, or lethargy, it is considered a medical emergency and veterinary care should be sought immediately.