Even arsenic, the poison of choice for many fictional murderers,...

Even arsenic, the poison of choice for many fictional murderers, is now close to qualifying as a micronutrient in animals. It seems that arsenic has a role in the metabolism of the amino acid methionine and in gene silencing (Uthus, 2003). Other work suggests that it has a positive interaction with the more important micronutrient selenium (Zeng et al, 2005).

In fact, if arsenic is essential for humans, its recommended daily intake would be little different from selenium, which is so important that evolution incorporated it into the rare amino acid selenocysteine—the crucial component of the antioxidizing selenoproteins that help to repair other proteins from oxidative damage. The recommended dose of selenium is 40 μg per day, whereas extrapolations from mammalian studies suggest that humans might need between 12.5 μg and 25 μg of arsenic. This is, to some extent, academic; a normal diet will contain 12–50 μg of arsenic in most parts of the world, but it shows that arsenic—the famous poison—and selenium—one of the most widely studied elements in the dietary context—could well have almost identical levels of nutritional necessity and toxicity.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2246629/