
Some wormers need to be given two weakly as they deal only with the adult stages or advanced larval stages.
Panacur deals with larval stages so monthly dosage is enough normally, but with puppies who are the main carriers of roundworms as they are passed on through the placenta and milk more frequent worming is needed.
Now I worm my bitches in pregnancy with Panacur which is supposed to minimise transference to the pups by 98%.
I was instructed by my vet to worm at 3 and 7 weeks, the one time I did it at 3, 5, and 8 I had problems, so have always done 3 and 7, and then 11, and monthly to 6 months.