Could it be allergy to the pads?

I work in a doctors office. We used to sell TENS units and what we found out was some people were alergic to whatever is on the conductive pads. There is something else you can use, but I don't remember what it is. I'll try to find out for you. I'll ask, but the doctor won't be back in town until Tuesday. I'll try to check with physical therapy to see if they know. Is there a brand name on the pads? Perhaps you can do a google search and find out more, perhaps contact the manufacturer of the pads.