There are hundreds of billions of dollars made by corporations in the business of manufacturing vitamins. And in this economy, when many people have lost their health insurance, people are doing and buying anything they think might protect their health out of sheer fear. Although vitamins are expensive and possibly price-fixed by some groups of manufacturers, people who can afford them are buying even more now than ever before. But if you don't have a diagnosed deficiency, do vitamin pills do anything for you? Is there a good reason for everybody to spend their money on vitamin pills, when there's no sign of scurvy, pellagra, beriberi, or other obvious deficiency disease?
Some vitamin pill formulations don't break down in your digestive system and just end up in your septic tank, and the jury is still out on whether some vitamin supplements work for people who are not very deficient. Most excessive water-soluble vitamins just overtax your kidneys when they have to filter out the extra. But if you're going to take supplements, you should know that you can definitely take too much of a good thing.
I haven't come across cases of bad things happening with large doses of C or B12, but large doses of some of the single B vitamins can cause nerve damage or heart destabilization. Overdoses of oil-soluble vitamins and some metals (minerals) can be especially dangerous, because the oils are stored in body fat and minerals stored in bones for a long time, and cannot be flushed out with water. Vitamin A overdoses notably cause liver toxicity, and wandering newcomers to Eskimo country who eat a slab of seal liver full of Vitamin A, instead of a tiny nibble, have been known to die pretty fast. Vitamin E used to be overused because it was falsely promoted as an aphrodisiac, before much was known about it; but while tiny amounts of it from your food are helpful to some body systems and may help prevent cancer, a link has been shown between taking more than 200 IU of Vitamin E per day and heart disease.
Please note also that we learn new things about nutrients all the time, and we can't necessarily predict every single thing we are going to need for survival and stuff it into a pill. Do the best you can to eat lots of grains and colorful vegetables and fruits, especially the dark ones. Drink lots of clean water to keep away gallstones and kidney stones, and help flush out water-soluble toxins. Eat what you consider to be a balanced diet. Get a tiny bit of sunlight to help the melanin in your skin make Vitamin D. If you still don't feel quite right, consult a doctor and a licensed nutritionist.