By their very nature, analgesics only work for a short period of time. Hence, to manage long term (or chronic) back pain, they need to be taken frequently. As a result, patients often develop a tolerance for the drug in question (an effect known as down regulation), and this leads them to seek stronger, or more frequent medication.
Side effects and drug dependency are also relatively common. For example, in a study conducted at the University of Toronto into the effects of long-term codeine use, 40% of patients were found to exhibit drug dependence, as defined by DSM-IV criteria (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition™). 67% sought help for mental health problems, most commonly depression. They concluded that ‘long-term codeine use is strongly associated with dependence’, and that ‘depression and depressive symptoms are common’.
Source: Journal of Clinical Psychopharmacology 19: 373-376 (1999); Dept. of Pharmacology, University of Toronto.