Infection Do anitbiotics make piggies poop more than usual?

Antibiotics usually has the opposite affect, it puts them off their food (bacteria upset in the gut) so they eat less, meaning the output is less. I wouldn't worry if he's producing a larger amount of normal (not runny/soft) poop, but I'd keep my eye on them to make sure they're staying in good form. As with humans, meds can have side effects, so while I'm no vet, I'd venture a guess that meds can give guinea pigs a side effect (excess thirst) too.