Dietary supplements are usually vitamins, minerals, herbs and extracts in the form of tablets, capsules or powders. Americans have been taking them since the early 1940s, when the first such products became available, the National Institutes of Health says. Now, more than a third of the people in the country use them. Many once-daily multivitamins contain all or most of the recognized vitamins and minerals, generally at levels close to the daily value, but people also eat foods that contain these nutrients, increasing the risk of overdose. The side effects can vary from mild to severe.