Antibiotics can be administered either orally, or by injection. Both methods have their pros and cons. Orals Oral antibiotics are typically in a suspension form that is flavored so that the guinea pig will willingly take them. They are measured out into an open syringe (a syringe without the needle), and the end of the syringe is place inside the guinea pig's mouth from the side (behind the front incisors). The antibiotic is then slowly injected into the guinea pig's mouth, so that he or she can lap it up as it comes out of the syringe. Oral antibiotics are fairly easy to administer, provided the guinea pig likes the flavoring of the suspension, but they are very hard on the stomach and digestive tract, because the antibiotic directly comes into contact with the bacteria in their digestive system. Injections Injections can be done at home if you have your vet show you the proper technique. Most antibiotics can be injected into muscle tissue, and many vets recommend that you choose ...
You can get SOME antibiotics at Tractor Supply. They usually have someone on staff who will know what you need. The ones I have used are added to their water. If you have trouble getting them to drink the "dosed" water, add tablespoon or so of sugar to mask the taste.