Short answer: give the dose as soon as you remember, then resume the monthly schedule. The rest depends on how late you are. Below, what to do at three days late, one month late, and two to three months late, with the American Heartworm Society's retest guidance.
The first thing to figure out is the gap, not the panic level. Find your situation in the table below and act on the row.
| Time since last dose | What to do | Test now? |
|---|---|---|
| Up to ~35 days (a few days late) | Give the dose now. Keep the original schedule. | No, unless your vet says otherwise. |
| 5 to 8 weeks (one missed month) | Give the dose now. Resume monthly. Mention at next vet visit. | Usually no, but ask your vet. |
| 2 to 3 months | Resume monthly immediately. Don't double up. | Yes — antigen test now, follow-up test 6 months later (AHS). |
| More than 3 months | Resume monthly. Call your vet. | Yes — antigen + microfilariae test now, retest at 6 months. |
Source: American Heartworm Society Canine Guidelines. Always defer to your own vet, especially in high-incidence regions or for puppies, seniors, or immunocompromised dogs.
Monthly heartworm preventatives don't work the way most people assume. They don't sit in the bloodstream protecting the dog all month long. They kill larvae transmitted in the previous 30 days. Give the chew on March 1 and it kills larvae the dog picked up in February.
Skip the March 1 dose, and February's larvae get to keep developing. Heartworm larvae need about six months from a mosquito bite to become adult worms detectable on a standard antigen test. That's why the AHS retest window is six months out, not next week.
That window is also why "double up next month" doesn't fix it. The dose can't reach back in time and kill larvae that have already matured past the susceptible stage.
Don't wait until "the original day" comes around. Give it now, with food, and check the dog actually swallowed the chew. Note the date.
From now on, this is your monthly anchor. If today is the 18th, the next dose is the 18th of next month. Set a reminder for it now while it's fresh.
Coughing, exercise intolerance, weight loss, and a dull coat can all signal a heartworm infection. None of these guarantee it — but they're worth a vet visit if you notice them.
An antigen test now establishes a baseline. A follow-up test six months later catches any larvae that matured in the meantime. The AHS recommends this exact protocol after any extended lapse.
The reason most owners miss a dose isn't carelessness. The interval is just long enough that the task drops out of memory between cycles. The vet's six-month supply doesn't come with a calendar. The pharmacy auto-refill notification looks like the last twelve auto-refill notifications. None of these systems follow up if the first nudge is ignored.
A simple recurring email reminder set to your dog's monthly anchor date catches it. See the full setup at the heartworm prevention reminder page, or set one right now below.
Set a reminder for the next monthly dose.
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Almost certainly yes. Most monthly preventatives keep working a few days past the 30-day mark. Give the missed dose now, then keep the original schedule. If it helps you remember next time, set a recurring reminder for that anchor date.
Give the dose immediately and resume the monthly schedule. Per the American Heartworm Society, a single missed month is a real gap in protection but rarely triggers the full retest sequence on its own. Watch for symptoms and ask your vet at the next visit whether they want to test sooner.
Yes, almost certainly. The AHS recommends a heartworm antigen test now and a second test six months later, since heartworm larvae take roughly six months to mature into detectable adults. Resume monthly prevention immediately while you wait — don't stop dosing.
Most labels and AHS guidance treat a few days late as low-risk and a full month late as a real protection gap. Give the dose as soon as you remember regardless of how late you are. Then set a reminder so the next month doesn't slip too.
If you're not sure, count the chews left in the box against the start date. If the math says one is missing, you probably gave it. If the math says one is extra, give it now. Going forward, log the dose by clicking the 'I did it' button in the reminder email — it removes the guessing.
Almost no. A single missed dose rarely causes infection. The risk grows with multiple consecutive missed months, especially in high-incidence regions. Resume the schedule, talk to your vet, and follow the retesting timeline if you've been off prevention for more than a couple months.
Free email reminder. Arrives a few days before each dose, with follow-ups if you don't mark it given. The reminder doesn't quietly vanish after one notification.
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