Drug abuse is the use of substances, such as medicines, which may be helpful when used as prescribed, at dangerous levels, to reap the "benefits" of side effects. For example, a traffic accident victim derives medical benefits from painkillers, but in an amount even slightly above the prescribed amount, painkillers may become (1) addictive, (2) self-perpetuating, (3) dangerous for their negative narcotic side effects, and (4) "gateways" to more dangerous drugs. Since drugs affect one's perception of himself or herself, as well as the decision-making process, a transition from drug use to drug ab-use may be subtle, and may even occur without one's volition.
The short answer, then, is to avoid using even common over-the-counter or prescription drugs, except upon the direct advice of a physician or when in obvious need (in the case of over-the-counter drugs), and then, only in the stipulated amount. For example, someone suffering from a horrible headache should not take 4 painkillers in 24 hours, when the directions say only 2 tablets should be taken in 24 hours. Any deviation may be seriously dangerous.
Similarly, non-prescription illegal "street drugs" are always dangerous, and should be avoided in all situations. Marijuana is an example of one such drug which carries serious consequences for its use. While television and movies may glamorize these illegal substances, there is nothing glamorous about them, and in real life, even slight use may be deadly.