Vitamin supplements can be a great addition to a balanced diet, to provide you with nutrients that you do not obtain from your diet. However, like anything else you put into your body, there is always the potential to consume too much of one or more vitamins. I don't know any studies that say that taking one multivitamin per day is bad for you. In addition, calcium supplementation is typically not harmful as long as it is done in moderation (one calcium pill per day). You may be reading studies that show that taking many multivitamins every day is not good for you, or you may be reading studies which show that taking certain herbal supplements can be harmful. It is true that some people take this supplementation too far and get overloaded with vitamins. This can be harmful to your health because certain vitamins in high doses can be toxic.
I suggest that you schedule an appointment with your primary care physician
. He or she can recommend a vitamin supplement that is most in line with both your current health and your diet. This way you can be sure that you are getting enough vitamins and minerals in your diet, without running the risk of getting too much. Good luck.