This helps in effectively controlling future flea infestations after the start of treatment and also eliminates existing. When a dog takes nexgard ® (afoxolaner), the active ingredient protects them by spreading throughout their system. Nexgard Chewables For Dogs Review - A Safe Flea Medication Nexgard spectra once given provides protection for an entire month from fleas and ticks.nexgard starts killing fleas in as little as four hours after administering and 48 hours for ticks.nexgard starts to work within just 4 hours and will kill all fleas that bite your pooch within just 8. How does nexgard work to kill fleas . Many reviews from veterinarians and consumers support this statement, with only a few nexgard reviewers reporting that it took an extra 24 hours for the nexgard flea and tick pill to start showing results. That active ingredient works for a month, after which your dog is vulnerable to flea infestations again. According to the product website, it does not take long for n...