The consumption of a daily multivitamin among people all over the world is dramatically increasing in recent years. Most of the people believe that if vitamins are not effective, at least they are safe. However, the long term health consequences of vitamins consumption are unknown. This study aimed to assess the side effects and possible harmful and detrimental properties of vitamins and to discuss whether vitamins can be used as safe health products or dietary supplements. We performed a MEDLINE/PubMed, EMBASE, Scopus and Google Scholar search and assessed reference lists of the included studies which were published from 1993 through 2015. The studies, with an emphasis on RCTs (randomized controlled clinical trials), were reviewed. As some vitamins such as fat-soluble vitamins (vitamin A, vitamin D, vitamin E), and also some of the water-soluble vitamins like folic acid may cause adverse events and some like vitamin C is widely taken assuming that it has so many benefits and no harm, we included relevant studies with negative or undesired results regarding the effect of these vitamins on health.

Our recommendation is that taking high-dose supplements of vitamins A, E, D, C, and folic acid is not always effective for prevention of disease, and it can even be harmful to the health.

Most people all over the world use a daily multivitamin for treatment or prevention of chronic disease. Uncontrolled advertisements about the vitamins in addition to wide availability of these agents result in high prevalence of their consumption among people. Although the effectiveness of the multivitamins and minerals is unclear, the prevalence of the use of these supplements in many developed countries is widened. The percentage of adults using any daily vitamin and mineral supplement has increased very rapidly in recent years. One third of adults and half of population aging more than 55 years report taking of at least one supplement per day. An increase in supplement sales has been seen since 1997, reaching to $18.8 billion in the United States in 2003.

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