Thanks a lot for your insights. I would rather be asked if there are any studies which identify the no hazardous dosages?

Also, I strongly don't believe if a substance destroys some organisms, inside a body will not do any harm. The right question would be rather how harmless/serious damage I can create? As u said the metabolism of a toxin, but also the environment that they got into is affected e.g. stomach, guts will it affect flora etc.
The scientist usually doesn't know what happens exactly but verifies trends, statistical correlations etc.
If you can please enlight us on some of these aspects, I will read with pleasure! Good luck & thanks for the info!