This is something that most medical professionals/educated addicts already know, but here is a little something from Dr. Alexander DeLuca (pain specialist), regarding opiates relatively benign status (when it comes to causing organ/bodily damage):
"A few patients experience myoclonus with opioid use. If discomforting, the condition can be treated with a benzodiazepine. Clonazepam is a good choice, because it is less sedating than other drugs in the same class. Opioids can sometimes cause urinary retention and increased bladder capacity due to increased sphincter tone. With high doses, peripheral edema may also occur as a result of antidiuretic hormone release, in which case a short course of diuretics is usually effective. **There is no evidence of any organ toxicity attributable to the prolonged use of opioids.**
The one common persistent side effect of opioid use is constipation, which is mediated by opioid receptors in the bowel. More than half of patients on sustained-release opioids experience constipation requiring specific therapy. It is important to get patients on a good bowel regimen as soon as possible and to teach them to adjust their bowel medications as needed. Many patients start out with a stool softener (e.g., docusate) and a mild fiber-based cathartic (e.g., senna). Osmotic laxatives such as milk of magnesia, polyethylene glycol, magnesium citrate, or lactulose will probably also be needed and may as well be started early. In contrast to stimulant laxatives, osmotic diuretics are safe and non-habit-forming. Patients have to find the dose that they can safely take nightly to produce a firm stool in the morning. Persistent constipation can be a serious problem requiring emergency treatment. I tell my patients to let me know if they have not had a bowel movement for at least three days. At that point, oral magnesium citrate or a sodium phosphate enema may be indicated...."
Although this is widely known in the medical community, a lot of recreational users (a lot- but by no means all) don't realize how safe (compared to other drugs) opiates/opioids are. Every drug has certain (side) effects on bodily processes/systems. Some drugs are much worse than others, of course.
But, constipation, urinary retention and the like are quite minor (and easily treated) side effects, in comparison to medications that can (and do) cause much worse things to happen to your body. Addiction issues are a whole different ball game, but even addiction in and of itself is not something that I (or most other medical professionals) would consider "toxic"...Maybe some of the things one would do, due to addiction, could be quite bad for you...but the addiction itself is not physiologically toxic (psychologically- sure; physically- no).
Just in case you didn't feel like reading all of that, the meat of the matter comes down to..."There is no evidence of any organ toxicity attributable to the prolonged use of opioids."