A drug is any chemical substance that produces a physical, mental, emotional, or behavioral change in the user. The development of drugs has given modern medicine many capabilities ... being able to sedate a patient before an operation, to numb the effects of pain, to produce medicine to fight infections, to stop that runny nose. But some of these drugs and many other manufactured drugs are abused and people become dependent on them. These abused drugs, including alcohol, tobacco, marijuana, and cocaine, often become addictive drugs and sometimes killers.