1. If your cat already has parasites, one deworming treatment is usually not enough to completely eradicate them. It's recommended to wait a week between treatments. Two to three consecutive deworming treatments are needed for complete eradication. However, some deworming medications are quite strong, so consult your veterinarian or seller before using them continuously. 
2. If your cat didn't have any parasites to begin with, and the deworming medication was only for prevention, then there's generally no need to worry. Cats usually won't have parasites after deworming. Furthermore, cats are generally not susceptible to parasites within one month of deworming. However, if more than a month has passed since the last deworming, parasites may become a possibility. Therefore, for safety, it's generally recommended to deworm your cat both internally and externally once a month to ensure they are free of parasites.
