It could be a small amount consumed just one time, or a slow and steady change over years. Eventually, every moment of every day becomes a mission for the next hit, high or fix. Your life is not your own, and the person you were or who you knew and loved, is gone.
Illicit drugs are illegal, extremely addictive substances like cocaine, methamphetamines, heroine, opiates or others. They are so powerful that many people become addicted, that is, physically dependent on them, often after just one use. Others get addicted by building up a tolerance to a drug, and therefore need more and more of it to feel its effects.