That's because they do it after at least decades of use and for a much smaller percentage (if opioids were used by the same number of people, we'd be knee-deep on corspes in the streets), and they don't affect nowhere near as much (if at all) the general functioning or mental health of their users.
Even the average user's health is not really that affected, unless they get C after decades of use. There are much worse offenders (like eating and a sedentary lifestyle).
Is that true? I am not denying it, but the opiate overdose rate is staggering.
Whether they are a fair comparison I'm not sure. It might be more accurate to say that while fentanyl can kill you, cigarettes make you live less long, rather than to say they kill you. No one drops dead from lighting a cigarette. Meanwhile, if it weren't for unreliable dosing due to black market pressures, I'm not sure regular, reliably dosed opiate usage would kill you slow the way cigarettes do.[citation needed] Anecdotally, I've noticed that ex junkies often look ten years younger than they have any right to.
US drug policy is not about harm reduction or life saving, and never has been.