Some wormers are POM (prescription only medicines). This means they can only be sold if a vet has examined the animal and prescribed that medication. For long term medication that is usually taken to mean having been seen for that condition in the last 6 months.
With worming and flea treatment a more sensible approach is to check if they have been seen and prescribed that medication before.
If not then a free flea check or weigh and worm is usually offered. Hell we may even administer the tablet for you!
It can be a pain to drag the cat in but at the visit I would usually suggest trying a new product that does fleas, ticks, eggs, round and tapeworm all together in one easy to use spot on. Much easier than giving the cat a tablet. POM only too though.