NexGard usually starts working the same day it is given, killing most fleas within about 8 hours and most ticks within about 48 hours after your dog eats the chew.

How fast NexGard works

  • Fleas
    • Begins killing fleas within 4–8 hours of administration.
    • Most adult fleas on the dog are typically killed by around the 8-hour mark.
  • Ticks
    • Ticks take longer; many products report effective killing within about 24–48 hours, with full tick-killing effect often quoted up to 48–72 hours after dosing.
  • Duration of protection
    • One chew protects against fleas and ticks for about 30 days, so it is usually given monthly as a preventative.

So in practical terms, NexGard is fast-acting against fleas the same day you give it, and you can expect steadily increasing protection against ticks over the first 1–3 days after the dose.

Why you may still see fleas

  • In homes with heavy infestations, new fleas can keep jumping onto your dog from carpets, bedding, and yard for days to weeks.
  • Those “new” fleas will still need to bite the dog to ingest the drug and then die, so you might see live fleas for a while even though the medication is working.
  • Environmental control (washing bedding, vacuuming, possibly treating the house/yard per vet advice) helps clear the infestation faster.

When to call your vet

  • If you still see many live, active fleas after several days on a correctly dosed NexGard schedule.
  • If your dog shows side effects such as vomiting, diarrhea, lethargy, or neurologic signs (tremors, seizures).
  • If your dog has a history of seizures or other conditions; your vet may adjust the flea/tick control plan for safety.

TL;DR: NexGard starts killing fleas within hours, with most fleas gone by about 8 hours, and ticks typically killed within about 1–3 days, with protection lasting roughly a month per chew.

Information gathered from public forums or data available on the internet and portrayed here.