Dr. Paul Corkery:

Primarily, it is the vet who determines how long the prescription will be. Member states put in place a system where it cannot be longer than a particular period. The practice in most countries depends on the pastoral arrangements. In a country like Ireland, it is usually determined by how long the products will be requited within the grazing seasons. The answer is that it is up to the vet, but it is up to the member state to determine for how long that will be.