**1. Introduction**

Dietary supplements are defined in the United States as products that contain one or more dietary ingredient such as vitamins, minerals, herbs, botanicals, and amino acids and are intended to supplement the diet [1]. In other countries dietary supplements are named differently including natural health products, complementary medicines, food supplements, and others [2]. Nonetheless, "dietary supplements" is a general term for products that mostly contain herbs, botanicals, proteins, and/or vitamins and minerals that are used with the intention to promote health. Despite the legal framework, dietary ingredients are often used and recommended for treating or preventing diseases. In this chapter, "dietary supplements" will be used as a general term to encompass several dietary ingredients.

Usage of dietary supplements has increased this last two decades [2]. From herbs, proteins, to vitamins and minerals, consumers are interested in selftreatment and preventing diseases [3]. Often using information from the internet to self-prescribe, many consumers believe that natural products are safe, while

many others avoid using these products because of the lack of an approval process by health officials in many countries. Many dietary supplements provide significant benefits to health [4]. However, the lack of guidance from health professionals can be problematic.

Dietary supplements are likely safe when used as prescribed [4, 5]. But, when combined with drugs and disease, these products can interact and cause side effects [6, 7]. Some of the steps to evaluate the safe use of dietary ingredients is to know their mechanism of action, clinical effect, and consumers' medical history. For example, an ingredient that induces liver enzymes will reduce the effect of a drug that is metabolized by these same enzymes. This can be life threating if the patient depends on this drug for normal function.

Due to the benefits that several of these dietary ingredients provide, it is important to evaluate their safety for wide spread recommendation. Particularly due to times of pandemic such as the coronavirus disease 2019 (COVID-19) [8], ways to prevent disease severity and to be used as adjunct treatments are needed. Several dietary ingredients have been reported to be effective against COVID-19 in review articles. For this book chapter, 30 review articles and meta-analysis were evaluated for the selection of the dietary ingredients herein discussed. The selection criterium was based on the number of articles that cited the ingredients as being effective as well as the commonality and accessibility of the ingredients across the globe. Vitamins and minerals were excluded due to their safety being extensively researched. Because COVID-19 severity is worse among patients with diabetes and cardiovascular disease, the safety use of these ingredients in the context of these comorbidities are presented here.
