What's good in small concentrations, but lethal in higher? What's a glaring red flag you're encroaching on a lethal concentration?

Acetaminophen/Paracetamol. The safe therapeutic dose is very close to the toxic dose. While most people don't intentionally overdose, at least not for treating illness symptoms, the problem arises when they take multiple medications that all contain acetaminophen, following the label for all of them can easily net you a toxic dose.

Chubbyemu video: https://www.youtube.com/watch?v=VSqrCgFMsCI