I once asked this question about a different medicine and my pharmacist said it could create a "dosage dump" whereby the pieces of a pill could have very different amounts of medication. It probably wouldn't make much difference if you're taking both pieces in the same day. But could make a difference with meds that are designed to create a fairly constant blood level like most antibiotics. The other factor to consider is that many drugs begin to degrade quickly upon being cut or broken. Any reliable, experienced pharmacist would know whether the manufacturer specifically warns against this. My pharmacist is a treasure trove of information including which drugs people are switching out of due to side effects and which drugs he's heard few complaints about and people are happily refilling. I get all pets' meds filled by him when possible, partly because his inventory turnover is faster than my vet's so the meds are just fresher. And his generics are all tier 1 where available. There are quality differences among generics no matter what the FDA says in my opinion.