Can I ask what you mean by backfiring? If you reach what's called bowel tolerance when you take vitamin C, then you decrease the dose a little and maintain it there; but usually it takes several grams for healthy people to get there, and more still when someone is ill.

Sometimes people claim they are able to take little or no vitamin C as it causes them problems, but usually these are people who are quite ill and whose systems are reflexively and protectively rejecting things whether they are healthy or not. I have spoken to a few people on this forum for whom this was the case, and they were unfortunately very ill. I've seen my ND give advice to people in this situation where they introduce a supplement and work up the dose very, very slowly. Something like vitamin C is necessary, especially for someone who is ill.

Don't knock megadosing any vitamins until you've done some research on them first, OK? Because what you personally believe or think is dangerous may not actually fit the facts. Sometimes a lot can depend on the kind of supplement you are taking and how it was derived, as well as if there are other nutrients present with which it works in synergy; also the state of your health, what kind of condition (if any) you are trying to treat, toxins present in your body, and other factors. These are reasons to say, as you do, that everyone is different and has different needs -- but they are not appropriate reasons for a blanket statement that megadosing vitamins isn't a good idea.

If you'd like to cite some studies here I'd be glad to read the info/abstracts and discuss them. I'm also happy to cite some of the info I've read myself if you'd like.