Cats should be dewormed externally once a month and internally once every three months. When choosing anthelmintics, you must go to a regular pharmacy to buy them, and follow the instructions to control the dosage correctly. Some cats will have diarrhea and vomiting after taking anthelmintics. This is normal and owners don’t need to worry. When deworming cats externally, be careful not to take baths for a few days before and after to avoid affecting the effectiveness of the medicine.