Drug addiction usually starts with a peer pressuring them into taking the drug, or they take them because they are told that it will help an injury that they have. Another way that it can start is to help with anxiety. If the drug does what they wanted it to do, they will continue to use them and that is how the addiction will worsen.
There are many changes that can happen to them over time if they continue to take the drug. It can destroy their self control to the point where they will do anything to get to the drug. It will make it harder to learn, make decisions, and remember things over a long period of time.
When on the drug, they have trouble talking. They can also stumble and lose thoughts, It impairs their vision and hearing too. Their eyes will become red and their handwriting will be very sloppy.