Drug abuse is the overuse or misuse of drugs for non-medical purposes. These can be illicit drugs such as speed, ice or prescription drugs such as Valium or common anti depressants such as Xanax, Codeine, Oxycodone (Endone), Dexamphetamine, etc.
There can be a variety of reasons for drug addiction including having an addictive personality, which is a predisposition to developing addictions. You could also have a parent who is an addict or have been exposed to drug addiction from an early age.
Addiction might also result from experiencing a severe physical trauma or injury or a severe psychological trauma such as the loss of a loved one. If you suffer from high stress levels or have a mental health condition such as anxiety or depression, you could also be more likely to become addicted to drugs.
The possibilities are many, but the common denominator is that if you turn to drugs to cope with whatever you think is wrong with your life, the cure often turns out to be much worse than the cause.