If at any time during treatment the cat's health deteriorates significantly, this situation must be addressed with a veterinarian. FIP almost never goes alone, and very often secondary infections may be associated that require the use of ATB, or there may be any other health problem unrelated to FIP. It is therefore very important to distinguish between the so-called relapse with deterioration due to FIP and other health problems. There is no place to panic and increase the dose immediately.
If a relapse occurs, discuss this with your veterinarian or your advisor during FIP treatment. The next course of treatment is quite individual.