What about the ones who aren't on drugs when it happens?
I'm not sure what you mean by this. Near death does not really mean much unless it involves oxygen starvation in the brain. Simply having one's heart stop is irrelevant if there is a mechanical method for pumping blood to the brain.
The effects of oxygen starvation are similar to the effects of some drugs. So I am having trouble understanding how you could have a near death experience without having something -- at least temporarily -- wrong with the brain.
Visions in the absense of any brain abnormality would be an entirely different matter. I don't have any pat answer for that, althought I can see why people would be really reluctant to talk about them. the first impression would always be that the person is nuts. But I'm not going to judge anyone without knowing them.