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

My medication. Lithium in small exactly precise doses according to each individual is helpful. But we have to take blood tests to check the toxicity.

We have to stay hydrated and be really careful if we are sick or vomiting

A warning signs is shaking (I tests my hands regularly), nausea and outcome is dying. Ha.