Arsenic—the infamous “king of poisons” used by the murderous Borgias—can poison humans and bacteria alike. But it doesn’t just poison bad bacteria–arsenic hurts the helpful bacteria that make up our gut microbiome, which protects us from E. coli and helps digest our food. Even worse, the microbiome may be harmed by a common method of mixing iron into water to remove the arsenic. Chinese researchers reported recently that mice which consumed arsenic-contaminated water with iron had healthier intestines than mice which drank only the water without iron. However, the healthier mice still had unhealthy microbiomes. Their gut bacteria had evolved methods for protecting themselves against the toxic metals—methods, unfortunately, which also protected them against antibiotics. The researchers cautioned that people drinking iron-treated water to avoid arsenic poisoning may develop antibiotic-resistant bacteria, making them vulnerable to dangerous infections.