At any given time of the day or night, some number of people are driving around under the influence of substances of unknown origin, unknown quality and unknown quantity. Many drug overdoses occur because the user falsely assumes that one batch of drugs is the same potency as the next. Without a kit to analyze samples, there's no way to know. Most hard-core drug users and addicts will experience jail before they ever see the inside of a clinic. Which segues nicely to the subject of addiction.
Some people are pre-disposed to addiction.
People who are naturally obsessive or compulsive can become addicted to any mood-altering substance or situation. Ironically, addiction can give some people something to focus their lives on, giving them a sense of predictability, control and purpose they otherwise lack. The addiction forces the addict into a set of behaviors that give their lives structure. In this sense, the addict's behavior isn't much different than that of the hourly employee. Predictablility, punctuality and fear driven by need join the addict and the worker at the hip.