Hi!
In many countries baytril is the only licensed antibiotic for guinea pigs. It is however comparatively harsher on the gut fauna than some others. In most cases all you will see is some softer than normal poos, but it can diminish the appetite and in some cases kill it completely.
Adverse reactions can happen to any antibiotics that are safe to give to guinea pigs; we see it most often with baytril because it is by far the most prescribed antibiotic for guinea pigs.
Please have probiotics or homemade 'poo soup' ready, as well as syringe feeding recovery formula so you can step in at any time.
First Aid Kit: Easily available non-medication support products for an emergency
Guinea Lynx :: Medications
Complete Syringe Feeding Guide (includes a list of non medical support care brands and products)
Can I please ask what the antibiotic is needed for?