Some folks say don't worm unless you have evidence of worms. You can take a fecal sample into a vet to check for worms. Others feel you should worm every six months. That's what i've been doing last couple of years. Pick times (like Spring and Fall) when egg laying slows down anyways, as you won't be able to eat the eggs for 14 days after last dose. But it's so easy for them to pick up worms, better to be safe than sorry.