Worms...

[pinkpixie83]

Hi don't know if anyone can help or offer advice.
I have an 18 week old pup, he's due his next worming tablet this Saturday, we've wormed him every 4 weeks since having him at home, and apparently he was wormed by the breeder, both times after the tablet we've seen worms in the poo following, more the first time than the second.
Last time he was really off colour on the Friday before the tablet on the Saturday, to the point I had him at the vets and everything but they made me feel like I was over reacting.
Today my friend who looks after him has said he seems quieter than normal and a slightly sloppy poo, although he seems fine to me, he was sleepy last night but I put that down to the heat.
Is it possible to never fully clear the worms even with the regular treatment? I think after this tablet they are talking about going to a 12 week worming schedule but I dont want to do that if hes not right.