Pennies minted after 1983 have a high amount of zinc, which is toxic to dogs. If a penny is ingested, the stomach acids will erode the copper coating and expose the zinc center. The zinc can then be rapidly absorbed into the system. Zinc toxicity results in a potentially fatal blood disorder in which the red blood cells are destroyed and the pet becomes anemic. Zinc toxicity in dogs may result from ingestion of as few as 1 to 3 pennies.

For more information, please read the article Zinc Toxicity.