From much personal experience as well as research plus just plain common sense: if one injestd any type of drug (or foreign substance) into the body, said body's chemistry is changed. There are many factors to consider, of course: your genetic profile, initial psychological state, quality of substance injested, etc. But inevitably if one continues to use, for example, opiates for a length of time much shorter than 50 years, manic depression (NOT the schizoid type - usually) will result. It should be classified as a type of manic depression but insurance companies would cause much jumping thru hoops for treatment. And this is just one example, which I use as it is so prevalent and applies to not just opioids, and also happens to be one I have done research. The possibilities of other chemical changes are as myriad as the types of opioids, cuts, country of import, etc. Extrapolate from there when you consider the spectrum of drugs.