Guinea pigs as a group are unusually sensitive to certain antibiotics, whether they are given orally or by injection. Potentially harmful antibiotics include ampicillin, penicillin, bacitracin, erythromycin, lincomycin, gentamicin, clindamycin, streptomycin, vancomycin and sometimes tetracycline. Interestingly, even certain antibiotics used topically may produce lethal effects.
The major way in which certain antibiotics cause reactions is by altering the normal microbial balance within the gastrointestinal tract. Once the normal intestinal microfloral balance has been upset, certain bacteria multiply to abnormally large numbers. The multiplying bacteria produce harmful chemicals and gases that can have lethal effects.
Certain antibiotics are directly toxic and do not alter the microbial balance within the gastrointestinal tract. These antibiotics should never be used in guinea pigs. Though injectable antibiotics can cause the problems described above, oral antibiotics are more often associated with them.
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Antibiotics should never be given to guinea pigs unless they are prescribed by your vet.
The antibiotics your vet prescribes will be based on what is best for the type of infection they are treating. Many antibitotics are not licensed for use in guinea pigs (this is common in most species which are not cats or dogs) so you may be asked to sign an off-licence consent form.
Some owners and vets like to provide the guinea pig with probiotics during a course of antibiotics to help replace any damaged gut flora.