I think that most of the people experiencing diarrhea and nausea are taking a solution that has too high a concentration of free ClO2 in it.
However, I will not totally rule out the effects of toxin die off.
I have very limited experience with this (only around 7 months), so keep that in mind. My observations have been that if a person is having problems at 1 or 2 drops, they are probably experiencing a toxin die off. On the other hand, if a person is ramping up and suddenly is having problems, they are most likely taking too high a concentration, or taking too much.