There is more to drug rehab than just getting off drugs.
Drug addiction is both physical and mental. These two aspects are equally important and neglecting to address either one is asking for failure in any treatment program.
Psychological addiction can begin very rapidly. In fact, many addicts claim their addiction began with the first use of their drug of choice. Technically, it is not possible to be addicted to something you’ve only used once. But part of addiction is a burning desire to use the addictive substance again, and that can happen in one use.
Physical addiction requires considerably longer to develop, but can be much more subtle and go completely unnoticed until one day the addicted person tries to quit and finds physical withdrawal symptoms appearing. Now the body is screaming for the substance. Those pains and discomforts will disappear with use of the drug, further strengthening the addictive stranglehold the drug has on the addict.