You might notice a change in how other chemicals and circuits of your brain function if you are on drugs for a long period. They could impair your judgement, ability make decisions, memory and ability to learn. It can be difficult to resist the urge of looking for drugs or to use them in ways you are unable to control when you have all these changes to your brain.
One example is if you regularly use opioids to relieve pain, you may develop a tolerance or even become physically dependent. You don't have to be dependent on something. A small fraction of people will become addicted even when opioids are given under medical supervision.
Tolerance, addiction, and abuse are all different. You either take more medication than you are advised to or you use a prescription from someone else. If you are looking to escape reality, feel better, relax or have a sense of accomplishment, then you might use drugs. You can usually change your bad habits and stop using entirely.