What is the best medicine to give a puppy for a bad infestation of intestinal worms?
The typical medication given to puppies for the most standard worms is Pyrantel. This is generally given every 2 weeks of a puppies life until they are 12 weeks old. This prevents and clears up most roundworms which are the most common types of worms found in puppies. It also is effective against whipworms and hookworms which are less common in puppies in most areas.
However, this does not protect against tapeworms. Tapeworms come from fleas and are typically found when the dogs are a bit older and look like grains of rice in the stool. Praziquantel is the treatment of choice for tapeworms.
There is more information about worms and their treatment on this webpage: https://www.vetbabble.com/dogs/health-dogs/worms-in-dogs/
All of this being said, it is important to see your veterinarian if you have an active infestation as they can have a serious health impact and sometimes warrant additional therapies.