NexGard typically begins killing fleas within about 4–8 hours after a dog swallows the chew, and it starts killing ticks within roughly 12–48 hours, with most ticks dead within 24–48 hours after administration. The product then provides ongoing monthly protection against fleas and ticks as long as dosing continues on schedule. A few notes to keep in mind:

• Individual variation can occur, and a heavy existing infestation in the environment may mean you still see fleas on the dog for a couple of days after treatment as fleas jump onto the dog and are killed after feeding.
• NexGard is a monthly treatment, and missing a dose should be corrected by giving the dose as soon as remembered and continuing the regular monthly schedule; do not double-dose.
• If you’re using a NexGard combination product (NexGard COMBO or NexGard PLUS), kill times for fleas and ticks may be described slightly differently in the specific product labeling, but the general onset of action is within hours to a day for fleas and within 1–3 days for ticks, depending on the product and infestation level.

If you need exact onset times for a specific NexGard product (e.g., standard NexGard vs. NexGard PLUS vs. NexGard COMBO) or for a dog with particular weights or health conditions, I can tailor the details to that product and your dog.