Activated charcoal is used in the emergency treatment of certain kinds of poisoning. It helps prevent the poison from being absorbed from the stomach into the body. Sometimes, several doses of activated charcoal are needed to treat severe poisoning. Ordinarily, this medicine is not effective and should not be used in poisoning if corrosive agents such as alkalis (lye) and strong acids, iron, boric acid, lithium, petroleum products (e.g. cleaning fluid, coal oil, fuel oil, gasoline, kerosene, paint thinner), or alcohols have been swallowed, since it will not prevent these poisons from being absorbed into the body.
Read more about activated charcoal, here.