"Substance abuse, also referred to as drug addiction, has been described as the continuous use of a harmful or potentially harmful substance for the purposes of sensory (i.e. your senses) or mood alteration that overtime may reinforce its permanent consumption (www.who.int). One of the factors most often associated with predicting one’s risk of future drug addiction is age. From childhood to young adulthood, people tend to be most susceptible to experimentation with drug use, however as we grow older, continued drug use begins to reflect future addiction. Yet even mild drug use can disrupt a young person’s life by negatively influencing their ability to relate with others (family, peers, etc.), maintain a sober composure over long periods of time, and it can even result in temporary memory loss depending on the type of drug and the length of use."
And not all drugs people use that get them into trouble come from illicit sources.