OK PM, but it's not quite that simple.
Not all poisons kill in trace amounts.

If you're running out of food, and there's native berries of unknown safety, it may make sense to just gradually introduce them into the diet.

If one berry, along with a full meal of certified OK food makes you deathly ill, don't eat a handfull of those berries.
But if they don't, try two next time; then 4, then 8, etc.

That method isn't perfect either.
Chris McCandless probably poisoned himself with some vegetation that blocked his absorption of other nutrients.

"when the bigots of this world have been privileged for as long as they have, to them equality feels like discrimination." shiftless2