You can buy Panacur pellets to mix with their feed but they're expensive. Injecting is much cheaper and you know for definite that each pig has had the correct dose. It's easy to do most pigs don't even notice.
Ditto agree - your vet will make up indivdual jabs for you if you don't have many pigs, one of the ivermectin group of wormers, Noramectin, Doramectin etc.
The withdrawal period is 56 days, but i take it you are worming breeding stock. Fatteners generally are jabbed when they are weaned and don't need anything else to slaughter at pork weight.
mandy