I don't take my multivitamins because I have an unhealthy diet, I have a healthier diet than 90% of the people I'm around on a a day-day basis, I just take them because why not? Are they doing me any harm? Nope. Could they maybe do some good? Yeah, perhaps. I don't see any down sides...
I'm afraid you are incorrect. There are PLENTY of acute vitamin-mineral toxicities, although they usually require high levels of ingestion over an extended period of time. The most notable are iron, vitamin A, and vitamin D, although many vitamins are known to have undesirable side effects. For example, excessive vitamin C consumption is known to cause diarrhea and high intakes of niacin (B3) have been known to cause flushing and liver damage, among others. hypervitaminosis A and D affect everything from the liver to the skin. Excessive vitamin D (although what defines 'excess' is of some controversy) is linked to arterial calcification, excessive calcium in the blood, and calcification of soft tissue, excessive bone deposition, etc. Folate (folic acid) is a vitamin among whose many roles is the methylation of DNA - in short, this means it inactivates certain regions of DNA and activates others. A certain class of chemotherapeutic drugs are folate blockers, meaning that they help to slow the progression of cancer by preventing/slowing the activation of certain genes. Flooding the body with folic acid (from supplements, which are especially absorbable) antagonizes the action of the drug. But what if you're not on the drug? Synthetic folate could act as cancer growth promoters, in effect worsening cancers that are in their early stages of development. This genetic interaction was one of the primary arguments against fortification of grains with folic acid started about a generation ago.
Two other well-known large-scale epidemiological studies (ATBC and CARET) either found that vitamin supplementation did not help, or was in fact associated with higher incidence of certain cancers. The studies were actually stopped prematurely after it was shown that the vitamins were causing HARM instead of helping. Granted, the subjects were mostly in high-risk groups (e.g. workplace asbestos exposure or current smokers) but the studies underscore the fact that vitamin supplements CAN cause harm, especially if taken for extended periods. Vitamins are not inert substances - they are highly bioactive, reactive compounds with a multitude of systemic effects that we are only beginning to understand.
There is also a potential problem with overuse of antioxidant supplements like vitamin C or E, which are the most popular, along with what I would call "secondary" antioxidants - vitamins which participate in antioxidant processes and help to maintain the oxidant-antioxidant balance in the body. Simply put, we are generally told that oxidation is a BAD thing, but that obscures the fact that certain white blood cells use oxidation as a method to kill pathogens and foreign invaders, and that oxidation is a way to induce adaptations in the human body. For example, exercise induces a form of oxidative stress.,The body adapts to this stress by inducing growth; as a result, your endurance/strength increases. Through adaptations and maintenance of antioxidant pools, the body is able to keep the balance of oxidants and antioxidants in the body constant.
But if you force large quantities of antioxidants into the body, you risk upsetting this balance, driving the reaction the other way. With lots more antioxidants floating around, the immune cells' oxidative bursts don't work as well and you inhibit your body's ability to adapt. There is also some laboratory evidence that reactive oxygen species (oxidants) may also serve a POSITIVE regulatory role in the body.
So what do you do? GET YOUR ANTIOXIDANTS/VITAMINS/MINERALS FROM FOOD. Food contains a gaggle of hundreds of different active compounds, many of which we poorly understand, particularly when it comes to interactions between all of these compounds. As I mentioned, our understanding of vitamins themselves is still developing. Supplements may have their place for specific uses, like the examples I've given in this thread, but the available scientific evidence does not support regular use, particularly not in the quantities that the general public consumes them. There is very little, if any, evidence that they help, and in contrast, a greater amount of evidence that they can cause harm. Besides, vegetables are 100% safe. If I can get my vitamins/minerals/antioxidants simply by eating more vegetables, why bother with the pills?