Flea medicine typically begins working within a few hours. Oral flea medications can start killing fleas within 30 minutes to 4 hours, with most fleas eliminated within 24 hours. Topical treatments usually start killing fleas within 6 to 12 hours, with significant flea elimination seen within 12 to 24 hours. However, complete eradication of fleas may take several weeks as the treatment also needs to break the flea life cycle, including eggs and larvae in the environment, which can take up to three months to fully resolve. For best results, all pets in the household should be treated simultaneously, and environmental cleaning is necessary to speed up flea elimination.