
I always use Panacur 10% suspension. Only had one bad reaction when worming at 2 5 and 8 weeks. Having gone back to previous vet advised protocol of doing them at 3 weeks and then 7 have had no more problems. I had wormed all the mothers during pregnancy from 6th week to 3 days post whelping so pups would have had very light worm burden and I suspect that worming them too often with nothing for the wormer to work on caused problems.
If you use just the old fashioned Piperazine you will need to worm more often as it doesn't kill larval stages only mature worms, whereas Panacur and one or two of the others kills immature life stages.
Also always weigh each pup and dose accordingly. With panacur it is 1/2ml per kg of bodyweight. I always round down. so a pup of 1 1/2 kg would only get 1/2ml and wouldn't get 1ml until it topped 2kg and so on.