Your not going to walk into a doctor's office say, "Hey doc, I have back pain" and they'll write you a script for demerol. Its not used all that often orally and is primarily used IM for severe acute pain. Relatively speaking, demerol is a pretty potent opioid and also has many interactions not found in other opioids which is part of the reason its not used on the same scale as others.
Codeine is a very weak opioid but more or less on par with tramadal as far as analgesia in the limited window they are both effective. This is all irrelevant though because it comes down to the discretion of your individual health care provider. They have drugs they like and dislike and there are protocols of what to use when. Furthermore if you go in there asking about shit like "demerol" by name you're probably going to get an odd look at best and a "drug seeking" label at worst.
If you want to ask a question, that's fine - but please don't lie and say you 'can't find much information' on two very common drugs. Between BL, wikipedia, drugs.com, rxlist.com, epocrates, erowid, nih, medscape and many other common websites, you could spend the next 80 hours reading about demerol and barely scratch the surface of the available information ...and that's the newer and less researched drug of the two.
