@JoelHarmon The problem with every poison is dose. Many substances occur naturally, especially in plants, that our digestive system can easily handle if the amount consumed is small enough. And "small enough" van actually be pretty much, as I described
here for caffeine. Similar rules apply for bitter almonds (marzipan), oxalic acid (unboiled, e.g. in sorrel), coumarin (tonka bean, ...