Hi and welcome
Tribrissen is another brand name for the antibiotic sulfamethoxazole-trimethoprim, usually referred to as bactrim in the USA after the original brand name or sulfatrim (or before that as septrin) in the UK. It is officially licensed for guinea pigs in the UK.
Antibiotics need several days to build up to optimal efficiency and then peter slowly out; it's not like painkiller that work instantly. It can take between a day or two to over a week to show improvement.
Please contact your vet if there is no improvement within a week or latest by the end of their course; they may need another course or another antibiotic. It is worth taking URI seriously because it can serious complications in the long term.
Switch from the normal weighing once weekly to weighing daily at the same time in order check on the appetite. The need to breathe comes before the need to drink and only thirdly the need to eat. In combination with an antibiotic that can sometimes impact on the gut microbiome, this is a double whammy.
If you wish to, you can support the bacteria in the gut (which can sometime be affected by the antibiotic as well) with probiotics; they are not a medication but classed as a food supplement.
Probiotics, Recovery Foods And Vitamin C: Overview With Product Links
Please contact us for further support on this your ongoing support thread if the weight goes down over 50g in the coming days (keeping the information on a developing case together will help us to tailor and individualise our support best).
If your guinea pigs have been bought within the last month, you can reclaim the vet fees from the petstore you got them from as part of your customer rights of having been sold damaged ware.
New Guinea Pig Problems: Sexing & Pregnancy; URI, Ringworm & Parasites; Vet Checks & Customer Rights
I hope that this helps you? I
