There are no risks or side effects of EWOT if done properly. The most common side effect is some minor drying of the sinuses and throat from the oxygen air, which can be remedied by drinking more water and staying well hydrated.
The research by Dr. Manfred von Ardenne has shown that with the amount of oxygen being received during the 15-minute procedure, the partial pressure and the concentration of the oxygen are well within the safe limits to avoid any chance of oxygen toxicity in anyone using the EWOT correctly.