If he is passing the hairballs in his feces, then that is actually the goal for any cat. Excessive grooming and seasonal changes can affect how much hair accumulates in a cat's digestive tract. But, to your question, I have no idea - I can't imagine Cerenia is causing him to 'collect' excessive amounts of hair in his stomach by reducing vomiting - technically, it should be passing out the 'other end'. You could check with the vet to see if they think that is a possibility, or if they have ever heard anyone else speculate on it.Maybe Cerenia does more harm then good then? Not allowing him to throw up the hairballs?