If you feel tired and run-down you probably need more vitamins and minerals

This is not generally true. We can feel tired and rundown for all sorts of reasons, like overwork, inadequate sleep, alcohol excess, and being upset or depressed. These factors are much more likely to account for how we feel than a lack of vitamins or minerals. If particular circumstances decrease our food intake or its variety then there may be a risk of essential nutrient inadequacy, but this will not matter in the short term (say, a few days) because of body reserves.

If some is good, more is not necessarily better! Vitamin deficiency may lead to lethargy, but vitamin excess does not necessarily produce vigour!

There are risks with excessive nutrient supplementation, so if it is used it should be at levels corresponding to the RDIs (see appendix 2) and for the shortest possible time, for example, just a few days or weeks.