I'm sorry, but I don't have no sympathy for smokers who get lung cancer who are told of the risks.
Basically I feel the same about the HIV comment too.
I wasn't saying any of that "excuses" anyone for taking risks. It's about why people take risks. I agree to some extent, but the point is that this pattern is in our brains. When there's something we want, and suddenly the risk seems to be significantly diminished, we're more likely to take it.
This is, apparently, true even if we actually have no way of knowing whether the risk has been lessened; the treatment could be ineffective, or the subject could be receiving a placebo (one has to wonder is participants understand experimental protocol at all). With vitamins, this could mean that people who take a lot of them start to become more relaxed with their diets and exercise. You will find people who have a similar attitude toward exercise: a significant portion feel it is unnecessary if their weight is healthy, and if they eat well.