Most cat deworming medications are available without a prescription, so there is no requirement to see a veterinarian before buying and administering them. However, since not all dewormers are effective against all parasites and the medications do have side effects, it is advisable to use them under a veterinarian's guidance.
Many pet supply companies sell common cat dewormers, such as ivermectin and praziquantel. These can be toxic in large doses and may cause side effects such as vomiting and diarrhea. It is important to know what kind of parasites the cat is infected with before administering medication. A veterinarian can determine the parasite by examining a stool sample. Kittens often have parasites but may need different dewormers than adult cats.