alt Hacker NewsFentanyl is actually much more toxic than the mycotoxins produced by mold (which I'm guessing is what you're referring to), and yet fentanyl is generally recognized to be a useful drug because of its therapeutic effects. If something has no possible therapeutic, recreational, nutritional, or culinary effect, while being toxic, it might be fair to characterize it as a 'pure poison'.
That's a good definition. Still, you can take it literally - anything small enough doesn't really harm you. and meanings can be stretched to support whatever conclusions you want.