Multivitamin/mineral supplements (MVMs) are widely used in many populations to maintain good health and for protection from different diseases such as cardiovascular disease, cancer, and cognitive decline. MVMs are recommended for pregnant women along with iron and folic acid to improve birth outcomes and reduce low-birthweight and miscarriage rates.
The US Department of Agriculture/Department of Health and Human Services 2010 Dietary Guidelines for Americans acknowledge that “supplements containing combinations of certain nutrients may be beneficial in reducing the risks of some chronic diseases when used by special populations,” yet they also state that excessive use of certain supplements has the potential to be harmful.

So, to study the safety of using MVMs, researchers conducted a literature search of randomized controlled studies involving supplementation with at least nine combinations of vitamins and three minerals at a maximum concentration of 100% of the dietary reference intake. Finally, it was found that only minor adverse effects (e.g., non-specific gastrointestinal symptoms) were reported in all studies. In particular, there were no significant differences between the treatment and placebo groups. Of the studies that reported adverse effects, none have revealed any serious effects of MVMs. And the use of MVMs within the range of dietary reference intakes will not result in excess intake, even when including the effect of food and fortified food, and does not increase mortality.

Despite a balanced and overall healthy diet, micronutrient gaps may occur from time to time, and consumption of MVMs has been shown to reduce dietary intake gaps and to improve measures of nutritional status without exceeding the dietary reference intake or tolerable upper intake levels. And MVMs are safe at short- and long-term physiological doses (100% of the dietary reference intake), whereas adverse effects may occur if high doses of single vitamins are consumed.

In conclusion, a rational management program of MVMs can help to improve the nutrient supply and overcome problems of inadequacy without concern for its long-term safety.

Biesalski, H. K., & Tinz, J. (2017). Multivitamin/mineral supplements: Rationale and safety. Nutrition, 36, 60-66. [Link]