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Here is our post-op care guide (from the top of our Caring for an Ill Guinea Pig Section, which is always worth a visit: Tips For Post-operative Care
Ideally, your boy should come home as if nothing ontowards has happened. Day 2-3 are usually the ones when there is a bit of very minor discomfort until the body gets seriously into healing, but it should not translate to loss of weight, and from then on in, you shouldn't basically notice that he's had an operation.
If things don't go to plan, please be aware that loss of appetite is the most common side effect after an operation; this happens often on the day after the op. Be prepared for round the clock syringe feeding from as soon as it happens - your good care can often make all the difference in the outcome. You can find further links for that in our post-op guide.
Also be prepared to look out for swellings of any sort in the groin area; abscesses are the most common complication (and they can happen as late as several months post-op), but hematomas and hernias are not quite uncommon. The vast majority of neutering ops thankfully goes without a hitch. I would strongly recommend to check the groin area daily for the first 2-3 weeks, then every second day for the first month and after that once or twice weekly.
If you are confronted with an abscess, please ask your vet for a stronger antibiotic than baytril if he is prepared to prescribe it.