Laurie (3piggles) have provided some good information about antibiotics that can harm guinea pigs
"There's a great, short article on Gorgeous Guineas about Vetrumex, a med prescribed for piggies with diahrrea. According to the article, it contains Lactose, which is harmful to piggies. It also doesnt' contain a probiotic, or good biotic, to work in the intestines and undo whatever bad things are going on in the intestines. It's good to add this to you list of meds not recommended for piggies, though often prescribed for piggies. At least it's a chance to ask the vet questions, should this be prescribed for your piggy."