What are the symptoms of selenium overdose? I just started with onyx mineral and it has 26 ppm of selenium in it. We tend to have selenium problems in my area, too much of it, but I thought I'd try this and see what happens. I desperately needed a better mineral and I couldn't find anything that could be ordered here that didn't have a lot of selenium in it. Keeping my fingers crossed, but I want to know what to look for so I can hopefully catch it early if anything happens.