There are varied reasons why demerol is not usually the first choice for pain control (i.e. other drugs are more effective).
For precautions please see http://rxlist.com/cgi/generic/meper_wcp.htm
For side effects please see http://rxlist.com/cgi/generic/meper_ad.htm
I personally have known people who were put on Demerol years ago, and not only did they not get proper pain control, but they had a very difficult time getting off the drug. I hope this helps.
Wishing you pain free moments, hours and days.