Drugs react in the brain to change the way the body feels. The brain is the master control center of the body and it sends messages to the rest of the body based on the signals that it gets from chemical messengers, or drugs. Most of the chemical messengers are actually made in the body to do specific jobs in the brain. But drugs, which aren’t made by the body, are chemical messengers that can send the wrong messages to the brain, which then sends the wrong signals to the body.