Panacur won't prevent, just kill active infestations so it's not something you want to use regularly. Some folks use it once or twice a year just in case there are worms but that doesn't stop them from picking up worms a week or two later if they are exposed.
I use and like Parazap but it's not a cureall by any means. It's not effective against flagellates and protozoans so there's lots of common critters it doesn't touch. I've found it to be really great with pinworms and hookworms though.
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