Are you sure you are talking about tapeworms and not roundworms??? Roundworms look like pieces of spaghetti, tapeworms look like tiny pieces of rice. Pyrantal Pamoate does not kill tapeworms, only roundworms and hookworms.
But, yes, it is very normal for puppies your guy's age to be loaded with roundworms, especially if they were not wormed at a much younger age. You need to repeat the drug every week to 10 days because not only does your guy have adult worms, he also has eggs which are hatching out into MORE adult worms. Often there is such a load of worms after the medicine that they can get stuck and the pup may need help getting them out.
If the puppy is more weak and thinner than he was prior to the worming I would contact the veterinarian again and make sure nothing else is wrong. But, be aware that all those worms in his stomach and intestines were making him look fatter than he really was.
Good luck with him!

Toto, CD, RN, CGC answered on 5/7/11. Helpful? / 0