Unless a healthcare provider has prescribed a multivitamin or supplement for you, you probably should not be taking one. First, vitamins and supplements are not regulated by the FDA (Food and Drug Administration). Thus, you may not be taking what you think you are or the correct amount. The highest quality supplements tend to be the more expensive ones, which are often passed over for those on a budget.
Secondly, vitamins and supplements will not make up for a poor diet. If you eat a healthy, well rounded diet, you are probably getting all of the vitamins and minerals you need. Have your medical provider test your levels to determine if you have any deficiency prior to taking vitamins and supplements.
Furthermore, excessive doses can cause harm or even death! With fortified foods and drinks, multivitamins and additional supplements, it can be easy to unknowingly over dose. Most water soluble vitamins are excreted in the urine when you have too much. However, several vitamins and supplements are not easily removed and build up causing problems. Over supplementing can decrease the absorption of some essential vitamins and minerals. Too much vitamin C or zinc can cause nausea, diarrhea or stomach cramps. Too much selenium can lead to hair loss, GI upset, fatigue and nerve damage and too much vitamin B can cause leg cramps. Exceeding the recommended dose of vitamin D may lead to serious heart problems. Excessive folic acid can mask vitamin B12 deficiency and lead to permanent nerve damage if left untreated. Toxic levels of iron can cause skin discoloration, enlarged liver or spleen, abdominal pain, congestive heart failure, irregular heart rhythms and insulin dependent diabetes.
This is not a complete list of all of the side effects and dangers of overdosing on vitamins and supplements, but this should get your attention to realize that these are not always safe and need to be considered with all of the other medications you take and food you typically eat. Sit down with your medical provider or a nutritionist to determine what supplements are necessary for you and what dose you need to take based on your deficiency and the diet you normally consume.