Because the fruit's seeds are small, it can be difficult to avoid ingesting one or two on occasion. Apple seeds, cherry stones and nectarine, peach and plum pits are considered poisonous if consumed in large enough quantities. They contain a form of cyanide, a substance lethal to humans. But lemons are not in the same family as these fruits; lemon seeds are not considered dangerous. However, they might bother people with digestive disorders, such as irritable bowel syndrome or diverticulitis.
The ASPCA says lemon seeds are not poisonous for dogs to consume, although they could also possibly cause digestive problems for canines.