^not necessarily...i don't know how the pro-drug action of codeine is affected in this sense (ie is the liver the main converter of codeine into morphine etc?) but generally people attempt different routes of administration in order to get the substance into the bloodstream faster and/or more efficiently.
drugs all have different bioavailability in different modes of administration - some are very active orally, and others not at all.
there are a lot of variables, and some people have preferred ways of administering drugs, such as sniffing, or in the case of some opiate users, intravenous injection. with codeine this is potentially deadly, hence the talk of intramuscular injection, or my tentative suggestion of rectal administration.
it's worth further research, in the very least. it's not something i would be interested in trying (without a glowing recommendation, that is!) because i would simply ingest it.
however, i definitely wouldn't IM codeine extracted from pills, and not because i have a problem with needles or anything - just not something i do in risky circumstances. if the OP is keen to do something other than eat them, plugging might be worth considering.
there's got to be info elsewhere on bluelight about such things, and if not, there must surely be someone on the internet who has tried plugging codeine.