Drugs that are used to treat infections. Examples of well-known antibiotics include penicillin, tetracycline, and amoxicillin. One of the first antibiotics, penicillin, was a by-product of a mold growing on bread. Today many antibiotics are synthetic, but some are still made using micro-organisms to produce the drug. Depending on the severity of the infection, antibiotics may be given by mouth or by intravenous injection.