It all depends on the opiate.
It is not possible to lump all "prescription opiates" into one category and compare them all against heroin. The fact is, there are a WIDE range of prescription opiates... almost every imaginable semi-synthetically produced, as well as fully synthetic (aka fentanyl), opiate that can possibly be created from the poppy has been formulated by pharmaceutical companies for pain management use.
Heroin is not used in hospitals or prescribed for pain mostly because of its stigma - people equate heroin with "bad, dirty hard drugs." But the fact of the matter is, heroin is an opiate just like any other, a semi-synthetically derived opiate from morphine - technically known as diacetyl-morphine. There are actually some countries which use diacetylmorphine for medical purposes, pain relief in hospitals. The only truly derived opiates (not synthetic in any way) from the poppy are morphine and codeine, which are modified to create all other semi-synthetics by pharm companies (don't blast me, of course there are many many more active chemicals in the poppy, these are just the MAIN, mostly concentrated, ones).
Anyway, to answer your question. It depends on which pain killer you are comparing to heroin. Both oxycodone and hydrocodone are semi-synthetic opiates derived from codeine. These are NOT more addictive than heroin. I would presume its safe to say that heroin will be much more habit forming than either of the two... ESPECIALLY hydrocodone, which only comes in formulations with tylenol in it, so snorting/IV is pretty much out of the question, as well as swallowing enough without destroying your liver before becoming addicted to it faster than heroin. That isnt to say hydrocodone isnt addictive, many people become addicted to it - it just isnt as addictive or reinforcing as heroin. Oxycodone, on the other hand, can be extremely addictive - even with the stupid OP formulation, one can still get just plain old generic 30mg oxycodone by itself... which can be IVed and snorted - there isnt really a rush to speak of when IVed, just a quick onset, but one cannot deny that it feels DAMN good and hits those pleasure centers in the brain quite nicely... and then wears off damn quick, causing the desire for another shot... aka addiction.
Even though it depends on the person, it is safe to say that heroin is MUCH more euphoric than oxycodone and thus can cause addiction much more easily. When IVed, 100% would say heroin will grab your ass so fast you wont know what hit you - compared to oxycodone.... Even when smoked/snorted, I'd say it would be still be easier to become addicted to heroin than oxycodone taken orally or snorted. But again, it depends on the person - BOTH with get you physically dependent pretty damn fast for sure though.
NOW, here is where it gets tricky. This is exactly why I said you cant lump all prescription opiates together and pit them against heroin...
The two semi-synthetic opiates that are derived from morphine are SO reinforcing and pleasurable that I would go so far as to say that they are more addictive than heroin. I'm talking about hydromorphone (Dilaudid) and oxymorphone (Opana).
Don't concern yourself with hydromorphone if you are not talking about IV opiate use - Dilaudid is pretty useless orally/snorted in terms of getting high. However, IV hydromorphone is probably one of the most pleasurable feeling any drug can give any person on this planet - it literally is a train wreck of a rush - it feels like a train of feathers is crashing through your body, thats how I describe it at least hah... its so pleasurable that its almost crippling and you have to sit down, and anything you ever thought of as euphoria is NOT EUPHORIA when compared to a dilaudid rush.... but then its GONE so fast... no legs to the high whatsoever - you are in heaven for 2 minutes and then maybe 5 minutes of a lingering feeling and then NOTHING, baseline. This extremely reinforcing feeling, coupled with its awfully short half life will have you screaming for another hit. this is so sure to cause addiction, even more so than heroin I would say... because heroin offers the user a rush, followed by hours of a nod/high... and of course, heroin smoked/snorted lasts quite a while.
Thus, I've got to say, personally, dilaudid (hydromorphone), a prescribed pain pill, is more addictive than heroin.
Now, onto oxymorphone (Opana) - This is the king of all opiates - its also derived from morphine. As I stated earlier, heroin comes from morphine also... that's why the pain killers that are synthesized from morphine are basically the only ones that can be compared with heroin in terms of addictive qualities. They are all addictive, and oxycodone sure as hell is addictive, but in general, those that come from morphine are sure to suck you down a hole of addiction faster.
Anyway, oxymorphone is the opiate that spiraled me into addiction... it is so unbelievably pleasurable and addictive I can't even explain in words. I was in horrific pain, so my case might be different... I rationalized a lot of my opiate use because of my pain, so I can't speak of how addictive it is from a "non-pain-user's" perspective, someone who only uses it for the high. But, when snorted, oxymorphone will have you nodding so hard from a low dose that you'll drool on yourself... wonderful euphoria. I IVed my instant release and snorted the extended release pills - so if you arent an IV user, I can definitely say that snorted oxymorphone feels just as good if not better than snorted/smoked heroin - thus, it is just as if not more addictive.
I myself began IV the moment I found myself in chronic pain with a nerve damaged, paralyzed arm (thank god its gone now!) ---
I have to say, oxymorphone IVed is probably the most euphoric of all opiates, HANDS DOWN. Not only is the rush the most pleasurable thing you will ever experience, but it has legs and will have you in a daze/nod for hours, much like then length of heroin, completely different than hydromorphone... the rush from oxymorph is much calmer than hydromorph, but its more pleasant... its hard to explain, but dilaudid is like a punch in the face of pleasure, while oxymorphone is like a massage of pleasure haha... Anyway, my point is, oxymorphone, when IVed, has a this slow warm rush up your body, crawling up your spine until it bursts in your head, washing pleasure all over your brain, sending you into a daze of bliss for a few hours.
In my opinion, oxymorphone feels 10x better than any heroin you could ever get your hands on - even straight from the lab, uncut heroin, couldnt compare to the feeling of oxymorphone... once again, this is an opinion!!! It is also, mg to mg, stronger than heroin - also, withdrawals come on sooner from oxymorphone than heroin - not to mention the withdrawals slightly more intense in my experience. All of these factors, make oxymorphone more reinforcing and thus I believe more addictive than heroin..... you find yourself enjoying the high more, needing to do it more often because withdrawal comes on faster, so you keep doing it, and of course addiction forms.
But with that said, both oxymorphone and heroin are SO pleasurable, its almost stupid to compare them - if you do enough of both of them, you will get your ass dependent/addicted to them.
Anyway, my point has been to show that heroin is not necessarily worse than all other opiates just because pharmaceutical companies make them and doctors say its okay to prescribe them. Opiates are opiates, and I believe that if heroin didn't have the stigma that it does in today's society, that there would be hospitals which would administer diacetylmorphine to patients in pain after car wrecks, etc. The sheer fact that oxymorphone is widely accepted by doctors says something, it is just as, if not more, addictive than heroin, yet it is totally acceptable in the medical community. I was in pain and they THREW tons of oxymorphone at me, more and more whenever i needed it - i mean, i was in HORRIFIC pain, but i was so addicted to it... so addicted. then when I would do heroin because I didnt have any of my opana, i would say "eh, this is alright, that that great."
So no, you are not some sort of leper, or nasty because you smoked heroin... if all they do is snort pills, that does not make them better than you. Opiates are opiates, and those kids who think you are fucked up for doing heroin are IGNORANT - how can they snort some addictive opiate as they tell you that heroin is bad?! Abusing all opiates is dangerous... they are all addictive. so tell them to fuck off!
thats my 2 cents... hope it was helpful.
Peace.