Maybe compounding the tablets takes care of the problem? Maybe guinea pigs don't have the same reaction? I have no idea.
I do know, however, that my pharmacist said it's really hard on their GI system. She didn't know about guinea pigs, but assumed it would be the same for them.
I look at it this way. Extra water is going to be good for them anyway, so why take a chance? Just give it with extra water.