Yes, it's true. I don't know of any statistics (wouldn't be surprised if there were many though) but it seems a lot of heroin/oxycodone addicts become so because of how well it works for coming off uppers....
A lot of people who get morphine/dilaudid shots in the hospital and have no experience with drugs, don't like it. It makes them feel itchy, sweaty, and uncomfortable. Drugs like crack & meth, are on the other hand, immediately likeable; the burst of euphoria is over-powering. Yet feeling comforted after the binge, is something out of their reach until they try an opiate; and that's where they move from being upper people to downer people.
It happens a lot this way it seems; because once the itchiness, sweatiness, etc. goes away from the histamine reaction and your body is no longer opiate naive; it becomes easier and easier to notice the high; I'd explain the difference in this way; dopaminergics like cocaine & amphetamine are reinforcing like the moments leading up to an orgasm; that mounting, increasing & intensely "into-it" feeling, which lasts several seconds at average in an actual orgasm but longer with cocaine and amphetamine; opiates on the other hand, heroin, fentanyl, etc., are that reinforcing feeling a split second after orgasm's intense effects, the release feeling that blends subtly out of the passion of the first effect and is therefore hard to distinguish out of context, that burst of relief that takes you out of 'caring about it' any more, and relaxes the body. This is the endorphin system being agonized, and it's less noticed because among the natural reinforcing processes of bodily function, exercise or post-orgasm, the 'rush' from endorphins is much shorter than the 'intense' into-it feeling which dopamine naturally occurs as.
So it's harder to consciously reflect onto the endorphin feeling and have as much experience with how much better it makes you feel naturally unless your body is severely strained, such as is the case with what uppers do to you... Once this is noticed about mu-receptor agonist narcotics like heroin though, it is much more addictive; because the drug feels so much more tranquilly wholesome and just "right", you don't feel disgusted with yourself as much as is commonly felt like during a coke or meth crash. There isn't as much negative reinforcement away from use from the side-effects; in fact the only negative side effect is, after habituation of opiates, from not having any more. You can sustain opiates as long as you have them: whereas uppers always eventually tear you down (never heard of a nonsleeping meth run over a month in length). Your body is stressed and on a non-euphoric overdrive. Thus why opiates are more longer term in getting addicted to, and a longer term addiction that holds you, in the long run.
