When chlorine dioxide in water was studied in humans, an extensive series of testing was done to monitor for adverse effects, including oxidative stress.
You can find the tests that were used under Evaluation Procedures in this document...
I will add that chlorous acid (MMS) has a much higher oxygen/reduction potential than chlorine dioxide, so the possibility of oxidative damage is higher. It all comes down to concentration. At 5 PPM there were no adverse effects.