There are no medical side effects to ERP. Doing ERP does not have any known dangers other than chance; when doing an exposure, there will be times, based on chance, that things will go wrong.
It is possible that while doing a driving exposure you will be in an accident. Or while doing an exposure based on not washing your hands, you may catch something and get ill. It is at these times that a person with OCD will likely have the greatest resistance to continuing the therapy and want to return to doing the rituals.
However, it’s important for a person with OCD to remember that chance affects everyone equally. Good things and bad things happen to all of us, and doing a ritual will not prevent bad things from happening.
In fact, it will lead to worsening OCD, which is obviously a bad thing. Therefore, it is important to recognize that doing ERP is a way to get you to think and behave almost like people without the disorder, to take risks and have fun and at times even be carefree.
It also means accepting that things will happen to you by chance, just as they do to everyone else, and recognizing that you can handle these things, no matter what they are.