Insulin overdose is an easy killer, done often in hospitals also air bubbles in blood stream. Big killer

cannot be done by mistake-but
a little known fact but anti-freeze is literally undetectable in food and in the body. A fireman and policeman were both killed by the same woman, Their wife. She married them and killed them. When the 2nd husband died, the first one's friends and family were very suspicious and demanded that the body of #1 husband be exhumed and it was discovered they both died from anti-freeze poisoning.

I killed a woman off with bella donna.(in a story!! please.....)

In the news a bunch of people died from a bad batch of meningitis vaccine