Welcome to the forum and I’m sorry to hear about this
Assuming all potential issues have been ruled out, including bladder stones, and several rounds of antibiotics have not been successful, the diagnosis often ends up being sterile cystitis.
Sterile cystitis is not bacterial so antibiotics will not work. They can suppress symptoms but they usually return once the antibiotics stop.
There is no cure for the condition but it can be managed with daily glucosamine, painkillers and diet management.
Strerile cystitis affects the glucosamine layer in the bladder which causes pain when corrosive urine is in contact. Giving daily the glucosamine to help protect the bladder helps. However glucosamine takes several weeks to build up enough in the system.
Do note that the oxbow food should only be one tablespoon per pig per day. Most calcium comes into the diet via pellets and through hard water so keeping them strictly limited is very important from a calcium intake point of view but also due to the other health issues too many pellets can cause.
The safe daily veg are lettuce, bell pepper, cucumber and coriander/cilantro. Corn is not ideal to be in the diet and should not be fed regularly in any event.
Vitamin c via supplements also not recommended. It can be given as a short two week course to help boost the immune system during a period illness. Piggies get all their vit C needs via a balanced diet. Supplementing with vit c above and beyond needs means their body can get used to artificially high amounts and can actually cause them to become unwell (with scurvy) if the level drops below that which their body had been accustomed to. Any piggy who has been supplemented with vit C long term would need to be weaned off very slowly and carefully so as to not make them ill.
I’ve added guides below which help to explain everything further
Wiebke's Guide to Pees and Stones
Long Term Balanced General And Special Needs Guinea Pig Diets
I have moved your thread to our monitored health and illness section where you can receive ongoing advice on this matter
I hope your piggy is ok
