The golden rule of electrical safety is: don't pass current through your heart. That's probably the easiest way to give yourself a heart attack.
Though despite the dire warning, I'd bet the level of juice this puts out is like on the safe side. Still, I'd rather not tempt fate and put two pads across your chest like they do on the box art. You can get away with placing two pads on each breast though. Generally the lower areas are safer.
The only other danger is getting burns from electrical shocks, but that generally requires really high voltages, much more than a few batteries can put out.