Medicines are usually grouped by their effects on the body and include commonly used drugs like aspirin, excedrin, and penicillin. Vaccines are prepared from dead or weakened germs and cause the immune system to produce antibodies to prevent diseases. Antibiotics fight specific types of bacteria and do not treat viral infections, while about 20 billion pain killers like aspirin are sold annually to relieve pain. Medicines must be tested and approved by the FDA before being sold and can have side effects if not taken properly.