A common (some maintain the most common) form of drug abuse. Because strong painkillers can be obtained very easily in either chemists/pharmacists or supermarkets, a drug abuser may turn to these drugs rather than other, stronger/more controlled substances.
There are many people who don't consider habitual painkiller usage to be abuse. In the same way that Nicotine and Alcohol are OK, painkillers are a part of our society.
Often the abuser will try to buy stronger and stronger painkillers to maintain the effect as both the mind and the body of the abuser become adapted to the drugs. If stronger drugs or professional help is not available, then the abuser will seek to obtain larger and larger doses of his/her favourite drug.
Eventually, as with all substance abuse, the abuser will badly damage his/her body. Fatal liver damage is a common result.