Appetite problem getting worse

My cat is on day 30 of molnupiravir and also eating kidney food because of kidney disease. These past 10-12 days i see him eating less and less. He’s now eating probably 1/4 of his daily requirement and i don’t understand why. He’s made quite a progress and acting pretty normal but he’s very uninterested in his food. I will talk to his vet in a few days but has anyone experienced something like this throughout their fip journey?