A Poison Ring and a Question about Poison

This bronze ring from medieval Bulgaria had a hidden compartment of the sort that is supposed to have been used for poison.

Which brings me back to one of my favorite questions: did ancient, medieval, or renaissance Europeans have any poisons deadly enough to kill with such a small dose? The harder I look into this question, the less I find. There were certainly rumors of poison and even somewhat credible reports of poison, but is there any reliable data?

Our ancestors certainly had poisons; Pliny the Elder said he knew of 7,000. They knew about arsenic, for example; but arsenic is not a reliable way of killing anyone in a hurry, and the fatal dose of most preparations is much more than you could fit in a ring.

They knew about deadly alkaloids from plants: belladonna or deadly nightshade, for example, and hemlock. Belladonna is quite dangerous, and it is possible to die from eating one leaf. The thing is, it tastes terrible; our dislike of bitter tastes evolved to help us avoid alkaloids like the ones in these plants. Was it possible to prepare a mixture from these plants that would be fatal without being obvious?