Opiates are completely non-toxic, much less harmful psychologically and physiologically than marijuana and most other drugs; ANY doctor will tell you this. The harm that follows upon opiate use attaches to the lifestyle that comes with opiates (poor diet, no exercise, etc etc), and the lifestyle follows strictly as a result of western laws. Opiates in themselves are not harmful at all, apart from their propensity to generate dependence. Pot, on the other hand, does damage your brain like all psychedelics.
Careful... these claims come from an ideal dreamworld, where product is pure and users are able to maintain at low to moderate doses, thus not having to deal with the negative lifestyle that follows a dreary drug habit in this naive law-burdened world, where users and abusers alike, are forced to compensate a healthy lifestyle for a less healthy one, due to the circumstances that are so often associated with the acquisition of illicit substance.
Opiates can be quite harmful, even when an otherwise healthy lifestyle is maintained. This is due to the profound effect that opiates have on a wide range of mental and bodily functions. Even if you are very social, eat well, and exercise all day long while maintaining an opiate habit, your circulation and breathing is still going to be somewhat shit when you nod off at night - and this will in the long run have a negative effect on your muscle and organ development, etc. On the contrary, there are cases, where people have smoked insane amounts of ganja daily, and lived to a very old age without a scratch. When you think about it, whether you live a really healthy lifestyle or not, the negative health effects are dependant upon the individuals mental/physical condition and lifestyle, ROA, purity of product, dosage, and tolerance. Even the healthiest individual will experience negative effects, at a high enough dose over a sufficient period of time. If product is not pure, contaminants will enter the body and might cause damage; whether it be smoked, injected, eaten, snorted, or stuffed up your ass; regular exposure to any foreign substance will have noticeable effects. The more you do, the more pronounced the effects; it is essentially impossible to completely compensate for the undesired effects through management of a healthy lifestyle. We must accept the consequences, and expect them, in order to limit damage. Only when you acknowledge the fact that opiates and other drugs will have some sort of negative effect, can you begin to consider damage control. So do not think that they are harmless, as you claim any doctor will tell you. The doctor makes these claims within a limited set of conditions.
My point here is that the notion of harmless opiates, is one of those things that gets all junkies convinced that they are doing the right thing, and that nobody has the right to take their DOC away from them; the laws are wrong, this world is fucked up and I'm not, without proof and first-hand experience first I won't believe what the doctor tells me, and yadiyadiya... I also used to think that I could handle opiates, and completely reduce damage by following a strict set of lifestyle guidelines. Likewise, I smoked a lot of ganja daily for years, and really enjoyed doing so. Amongst other things, I wanted to prove to myself and others, that it was possible to attain a university degree and stay healthy, even though I was a total pothead and psychonaut who could use various substances without feeling negative effects. I wanted to prove everyone wrong who didn't have faith in the idea, I wanted to prove the laws wrong too. And I wanted to prove that it was possible to be a responsible heroin user. Eventually, I ended up proving myself wrong, and realized that there was no reason for me to try and prove this sort of thing to anyone, before I accepted the consequences, and learned to deal with them in a responsible and reliable manner.
Even though damage from opiate use might be minimal on a strictly physiological level, how has it affected you on an emotional level? Your family relations? Do you still have the same loving friends, that you had before you tried to prove to yourself that you could live smacked out without hurting yourself or anyone else? The emotional turmoil that comes as a result of prolonged drug use, both in terms of opiates, cannabinoids, and a wide range of other psychoactive substances, also has a remarkable effect on mental and physical health. So before you convince yourself that you can get away with opiate use without any harm done, think twice, and think about all the factors that are in play - just because the substance might have passed through you with no toxic metabolites, don't think that you don't collect baggage and burdens along the way that are directly related to the use of either opiates or cannabinoids. And don't trust all doctors, in the end, they are just people like you and me.
EDIT: People, please don't take this post personally. I just wanted to express some thoughts based on my own experience, in terms of justifying an opiate or cannabis habit with health effects in mind.