NexGard kills ticks by poisoning their nervous system from the inside out. The active ingredient, afoxolaner, circulates in your dog’s bloodstream. When a tick bites and begins feeding, it ingests the compound, which blocks a critical nerve signal in the tick’s brain, causing uncontrolled hyperexcitation and death. Most ticks die within 24 hours of attaching to a treated dog.
How Afoxolaner Attacks the Tick’s Nervous System
Afoxolaner belongs to a class of compounds called isoxazolines. It targets a specific channel in nerve cells that relies on a neurotransmitter called GABA. In a tick’s normal nervous system, GABA opens tiny gates that let chloride ions flow between cells, which calms nerve activity and keeps everything running smoothly. Afoxolaner binds to these gates and locks them shut.
With those calming signals blocked, the tick’s nervous system goes into overdrive. Nerve cells fire uncontrollably, muscles spasm, and the tick loses the ability to function. This prolonged hyperexcitation is what ultimately kills it. The compound works at extremely low concentrations, blocking these channels with nanomolar potency, meaning even tiny amounts in your dog’s blood are enough to be lethal to a feeding tick.
One important detail: afoxolaner binds at a different site on the nerve channel than older pesticides like cyclodienes. That means ticks that have developed resistance to those older chemicals are still fully vulnerable to NexGard.
The Tick Has to Bite First
NexGard is not a repellent. It doesn’t stop ticks from landing on your dog or even from attaching. The tick must actually bite and begin feeding on blood to ingest afoxolaner. This is the key distinction from topical repellents or tick collars that aim to keep ticks from biting at all.
After your dog swallows a NexGard chew, afoxolaner is absorbed through the gut and reaches peak levels in the bloodstream within 2 to 6 hours. It then stays circulating at effective levels for weeks, with a half-life of about 15 days. That sustained presence is what gives the product its month-long window of protection.
How Quickly Ticks Die After Attaching
Speed of kill matters because disease-causing pathogens like the Lyme disease bacterium typically need a tick to feed for 24 to 48 hours before transmission occurs. NexGard’s goal is to kill ticks faster than that window.
In a study on European castor bean ticks, afoxolaner killed 93.4% of an existing infestation within 12 hours of treatment and reached 100% kill by 24 hours. For new ticks that attached in the weeks after treatment, the 24-hour kill rate remained strong: 100% at day 7, 99.4% at day 14, 96.6% at day 21, and 91.2% at day 28.
The 12-hour numbers tell a different story, though. For ticks that attached a week or more after treatment, only about 37 to 77% were dead at the 12-hour mark. At 8 hours, efficacy was even lower, sometimes dropping below 20%. So while NexGard reliably kills ticks within a day, it is not an instant kill. You may still see live ticks on your dog in the hours after they attach, even when the medication is working exactly as intended.
Does Efficacy Fade Before the Month Is Up?
Afoxolaner levels in the blood decline gradually over the dosing period, and this shows up in the numbers. A comparative study on blacklegged ticks found that 24-hour kill rates dropped below 80% starting around day 21 and fell to about 72% by day 28. By day 35, a full week past the recommended redosing date, efficacy was down to roughly 70%.
This is why consistent monthly dosing matters. If you stretch the interval between doses, you’re relying on declining drug levels that may not kill ticks fast enough to prevent disease transmission. Staying on schedule keeps afoxolaner concentrations high enough to deliver reliable 24-hour kills throughout the month.
Lyme Disease Prevention
Two FDA studies tested whether NexGard Plus could kill blacklegged ticks fast enough to prevent Lyme disease transmission. In both studies, dogs received a single dose and were then exposed to wild-caught ticks carrying the Lyme bacterium 28 days later, the tail end of the dosing period when drug levels are at their lowest.
The results were striking. In both studies, 100% of ticks on treated dogs were dead by day 5 post-infestation, and not a single treated dog tested positive for Lyme disease, either by blood antibody testing or by skin biopsy, even 104 days after exposure. Every untreated control dog that was exposed to infected ticks did become infected. The takeaway: even at the end of the dosing cycle, afoxolaner killed ticks before they could transmit the Lyme pathogen.
Which Ticks NexGard Covers
The original NexGard formula is FDA-approved to treat and control American dog tick infestations. The newer NexGard Plus formulation expanded coverage to include blacklegged ticks (the primary carrier of Lyme disease in the U.S.) and lone star ticks. For lone star ticks, studies showed 95.6% or greater effectiveness within 72 hours of attachment over a full month. Blacklegged ticks hit that same threshold within 48 hours.
NexGard does not target every tick species equally, and it is not labeled for use against all tick types. If you live in an area with a specific tick concern, knowing which species are common in your region helps you gauge how well NexGard matches your dog’s risk.