^ you have your last point for starters - getting an accurate dose is impossible and guessing via this route can be fatal. Next, as has been repeatedly mentioned, you have the danger of the histamine reaction from IV'ing codeine and certain other excessively histaminergic alkaloids which can produce pulmonary edema and other complications. Furthermore, if you try to micron filter solutions that contain way too many large particles, if the solution is too thick, if you try to force it through, etc. you risk breaking the filter slightly allowing larger particles through and not even noticing defeating the whole purpose of doing such which is why you should always filter with cotton or something else before micron filtering (sorry if this is common knowledge - to some it is and some it isn't).