Drug metabolism is the chemical alteration of a drug by the body. Some drugs are chemically altered by the body. The substances that result from metabolism may be inactive, or they may be similar to or different from the original drug in therapeutic activity or toxicity. Some drugs, called prodrugs, are administered in an inactive form, which is metabolized into an active form. The resulting active metabolites produce the desired therapeutic effects. Metabolites may be metabolized further instead of being excreted from the body.