The brain is composed of millions of interconnecting nerve cells, called neurons. In order for the brain to function so that a person can think, move, or feel, these neurons must communicate. They do so by receiving, processing, and sending chemical signals called neurotransmitters. When a neurotransmitter is released from one neuron it binds to another at a specific site called a receptor, much like the way a key fits into a lock. If the key fits, the receiving neuron will be able to process the signal and send it on. A drug is any substance that, once inside the body, changes how the body works. Drugs can be swallowed, sniffed, inhaled, injected, absorbed through the skin, or dropped into the eye. No matter the route of administration, most drugs will eventually travel through the circulatory system and reach the brain. In the brain, drugs can affect almost any step in the communication between neurons by increasing or decreasing the amount of neurotransmitter that reaches the receiving neuron. A drug can also bind directly to a receptor in place of a neurotransmitter. In the case of drug abuse, the interference in nerve communication often causes a temporary pleasurable sensation. With drug addiction, a person craves the pleasurable sensation produced by a drug and compulsively uses it despite its negative consequences. The use of certain kinds of drugs may also produce a physical addiction or dependence, meaning that the drug must be present for the body to function normally. In this case, when the drug is withdrawn, the user may experience mild to severe effects ranging from nausea to death. There are many ways to treat a person with drug addiction, and a physician-supervised program is the most appropriate and safe type of management.