Go on to any of the online vet suppliers and there will be a guide to buying prescription medicines - I'm fairly sure the vet will write a prescription for several months at a time but I believe by law they have to see the animal every few months. I have had 'country cats' over the years who hunted but still didn't worm them anything like every month (or is it one of the wormers that you actually give every month, like some of the flea treatments, rather than just an ordinary wormer that would usually be given every 3 or 6 months?)