If your vet prescribes a medication you can ask for a prescription.
If your pet needs long term medication your Vet will want to assess the suitability of the medication so they will start with a short term prescription. You can then ask for a repeat prescription which if not a human prescribed drug (yes your pet will sometimes be prescribed human medications!) you can ask for a prescription which is valid for six months. For human type medication you can only get a prescription for a month.