In the past, there was no effective treatment for FIP but thankfully times have changed and there are antiviral medications available with good results in treating FIP. These drugs are legally available in many countries and have been shown to be safe and effective (through clinical studies), although they can be expensive.
Current recommendations are to treat for 12 weeks to allow full clearance of the disease. This can seem a long time but gives cats the best chance of recovery. It may be that shorter courses can be recommended in the future but for now 12 weeks of treatment is required. Different types of FIP require different amounts (dosages) of antiviral medication, FIP cats showing neurological and/or ocular signs being treated with higher dosages. However, there is variation in how cats respond, and adjustments may be needed during the treatment period. If a cat is not responding as expected, the amount of antiviral medication may need to be increased.