In the dim & distant past when i were a lad
worming was a major operation - you had to starve the dog or cat for a day, force the worming syrup down their throats, the follow them around to clean up the poos/vomit to burn it as it happened. And believe me -IT HAPPENED.
(Sorry, not nice I know). In those days you did it twice a year, probably couldnt face it more often
. Nowadays things are much easier, for me & the dogs & cats, but the makers of these new, more efficient wormers say they should be used every 3 months. I cant help thinking they want to sell me as much of the stuff as possible, worms and dogs/cats havent changed their lifestages or anything. SO I still do it every 6 months except if anybodys pregnant (human or animal) or very young (again human or animal) . Nobody seems to suffer & I havent seen a worm in years (GOOD!!!) . Guess Im just cynical or something. But I AM REALLY GRATEFUL for the new, modern wormers - the good old days, heh!!