I'll just requote this as it sums it up fairly well:
"Actually, there is not much technical formulation difference in most legal vet. versus human grade drugs of similar strength. Occasionally human injectables will have different buffering for pain, but better vet drugs are buffered too.
Per a vet drug salesman, the difference comes mainly in QC, packaging, and licensing. She says some drug companies import materials and are only licensed to do vet. grade. Other companies may make/import only human grade. But, there are at least two companies she knows of that use the same drug, and package it differently for the human and vet markets. The labels have different company names on them too. So is the vet grade really human grade???? I would say so."