Heartworm Medication Schedule Calculator

The next monthly dose falls on the same date a month after the last one you gave

A monthly heartworm preventive, whether chewable or topical, is due again on the same date one month after the day you actually gave the last dose. If you gave it on October 6, 2026, the next dose is due Fri, November 6, 2026, and a year-round program means 12 doses in 12 months. If you gave it on the 31st and the next month has no 31st, the dose moves to the last day of that month. So after January 31, 2026 comes Sat, February 28, 2026, and then Tue, March 31, 2026 again.

Enter the last dose you actually gave and the dosing interval, and the calculator counts the days to the next dose and lays out the next 12 months month by month, like a medication sheet. If your vet set a prevention period, months outside it are skipped; if you add your most recent test date, it also shows a reference date for the next annual test. The next 12 months of doses can be saved as one calendar file (.ics) with reminders.

Why it pays to calculate dose dates ahead of time

One late dose can leave a gap in protection. The American Heartworm Society (AHS) tells owners that if you miss just one dose of a monthly medication, or give it late, your dog can be left unprotected. Yet a once-a-month rhythm is easy to lose track of.

• Due date versus the day you gave it: if the dose was due on the 6th but you gave it on the 10th, count the next month from the 10th. Counting from the due date changes the real gap: after a late dose the next one comes sooner than a month, and after an early dose it comes later than a month.
• Starting on the 29th, 30th or 31st: short months have no such date, so the dose moves to the last day of the month and goes back to the original date the month after.
• A label that says 30 days: giving the dose on the same date each month and giving it every 30 days drift apart by 5 to 6 days over a year (6 when February 29 falls in between).
• A dosing period set by your vet: if the period is April through November, you need to know the last dose of this period and the first dose of next year's so you do not miss the restart.
• Sharing the job: a shared link or calendar file keeps everyone in the household on the same schedule and makes it clear who gave the last dose.

How the dates are calculated

1. Same day monthly: add 1, 2, 3 months to the date of the last dose actually given. If that month has no such date, the last day of the month is used, and the next month returns to the original date (Jan 31 → Feb 28 → Mar 31).
2. Every 30 days: add 30, 60, 90 days to the last dose date. In months with 31 days, this can occasionally put two doses in the same calendar month.
3. 6-month or 12-month injection: add 6 or 12 months to the date of the last injection. There are no doses in between.
4. Date set by vet: the date you enter is used as is, and no later dates are added.
5. Prevention period: if you choose a period, only due dates from the first dose month through the last dose month are kept and the other months are skipped. A period that crosses the new year, such as November through March, works too.
6. Test reference date: your most recent test date plus 12 months.
7. When a dose is overdue: the tool does not slide the date or invent a new one. Later cells stay marked as to be determined, and entering the day you actually give the next dose restarts the count from there.

Basis Days left = next due date − today (dates only) · Doses in 12 months = due dates in the year after the last dose · AHS 2024 guidelines and owner pages, CAPC guidelines and DailyMed labels were checked on 2026-10-18.

Same day monthly and every 30 days drift apart by 5 to 6 days a year

Suppose you gave a dose on January 15, 2026. With same-day monthly dosing the gap between doses changes from 28 to 31 days; with 30-day dosing the gap is constant, but the dates creep earlier until the 12th dose lands 5 days sooner.

The AHS guidelines note that all oral and topical macrocyclic lactone preventives are labeled for a 30-day dosing interval, and that beyond that interval efficacy against late L4 larvae declines and becomes unpredictable. The Heartgard Plus label says efficacy can be reduced if the interval between doses exceeds a month (30 days), so the chewable should be given on or about the same day of the month. Follow the label of the product you use and your vet's instructions, and pick the same method in the calculator.

Here is how dates that a short month lacks are handled.

Year-round prevention versus a dosing period

The AHS recommends that veterinarians prescribe year-round preventives, and the Companion Animal Parasite Council (CAPC) says all dogs, regardless of lifestyle, should be on year-round heartworm prevention. In the United States that is the standard starting point.

The AHS guidelines say prevention is most effective year-round, but if a seasonal schedule is chosen, dosing must begin at least one month before heartworm transmission is expected to start and, depending on the product, may need to continue for up to 6 months after transmission typically ends. Outside the U.S., or when a vet sets a window, the logic follows mosquito activity. A document prepared by the European federation FECAVA together with ESCCAP and ESDA recommends year-round prevention in southern Europe, while in endemic areas of Central and Northern Europe it should ideally start less than a month after the mosquito season begins and can stop one month after it ends. If your vet uses a window like that, choose “Set months” and enter the first and last dose months. With April through November and a last dose on October 6, 2026, the last dose of this period is Fri, November 6, 2026, the next period starts Tue, April 6, 2027, and you give 8 doses in 12 months.

If a dose is overdue

Once the due date has passed, the calculator shows only how many days have gone by and does not make up a new date. What to do depends on the product and your dog, so this is what the source documents say.

• AHS 2024 guidelines: when prevention has lapsed or the product is changed, test for antigen and microfilariae before starting, and retest 6 months later. After that, test one year after the first test and annually, which means three tests in the first year.
• AHS owner page: talk with your veterinarian, restart your dog on monthly preventive right away, and retest 6 months later.
• Heartgard Plus label: if a dose is late, whether by a few days or many, give it immediately and resume the recommended schedule, which minimizes the chance of adult heartworms developing.

Once you have talked to your vet and given the dose, enter that day as the last dose given or tap “I gave it today,” and the next dates are recalculated from it.

When to test

The AHS recommends annual screening for all dogs over 7 months of age with both an antigen test and a microfilaria test. The guidelines explain that antigen can be detected about 5 months after infection at the earliest and microfilariae about 6 months after at the earliest, which is why dogs under 7 months do not need a test before starting.

The calculator's test reference date only covers the first row, every year. For the 6-month retests in the other cases, write down the date your vet gives you.

Injectable prevention intervals

The AHS guidelines describe prevention as several drugs given monthly, either orally or topically, or given by injection at 6-month or 12-month intervals. The U.S. labels say ProHeart 6 is not to be given more often than every 6 months (dogs 6 months of age or older) and ProHeart 12 not more often than every 12 months (dogs 12 months of age or older).

Choose “6-month injection” or “12-month injection” and the calculator shows the same date six or twelve months after the last injection. If your vet gave a different date, choose “Date set by vet” and enter it.

In the United States

Heartworm disease has been diagnosed in all 50 states, according to the AHS, which is why its guidance is not limited to a single region. The AHS campaign line “Think 12” sums it up: test every 12 months and give prevention for 12 months of the year. The CAPC guidelines, updated 2026-01-14, likewise call for year-round prevention for every dog, whatever its lifestyle.

For puppies, the AHS guidelines say to start no later than 8 weeks of age. If you start after that, test 6 months after the first dose and annually thereafter. How old a dog must be for a particular product is on the label: the ProHeart 6 label says 6 months of age or older and the ProHeart 12 label says 12 months or older. Follow your veterinarian's prescription and the label.

Common mistakes

• Entering the due date as the last dose: if you gave the dose late and enter the due date, the next date comes out earlier than the real interval. Always enter the day you actually gave it.
• Mixing “a month” and “30 days”: using the same date one month and 30 days the next makes the dates jump around. Pick one method.
• Letting the restart slip: with a dosing period, passing the first dose month means a stretch with no protection. Check the first dose date of the next period under “Set months” and put it on your calendar.
• Treating an injection as a monthly schedule: after a 12-month injection, a monthly schedule only piles up reminders you do not need.
• Saving a changed schedule on top of an old one: when a dose date changes, delete the events you saved earlier and then save the new file, or you will get two reminders.

Limits and cautions

• It only calculates dates. Product choice, dosing by weight and warnings for breeds sensitive to certain drugs, such as Collies, are not covered.
• It does not judge when or how to restart after a missed dose, or whether a test is needed. Your veterinarian's advice comes first.
• Prevention periods can differ by region and from year to year. The calculator uses exactly the months you enter.
• The test reference date assumes annual testing. Puppies and dogs whose prevention lapsed follow a different testing schedule.
• Calendar reminders behave differently in each calendar app. The site does not send reminders or store your dosing history on a server.

Sources

• 01American Heartworm Society: Canine Heartworm Guidelines 2024 (PDF)Year-round prevention, 30-day labeled interval for oral and topical products and the decline in efficacy beyond it, injectable 6- and 12-month intervals, starting by 8 weeks of age, annual testing over 7 months of age, testing when prevention lapses
• 02American Heartworm Society: Heartworm Basics“Think 12,” heartworm disease diagnosed in all 50 states, a missed or late monthly dose can leave a dog unprotected, testing timelines, retest 6 months after restarting (checked 2026-10-18)
• 03CAPC: Heartworm GuidelinesUpdated 2026-01-14. All dogs, regardless of lifestyle, on year-round prevention
• 04DailyMed: Heartgard Plus labelUpdated 2023-10-19. Efficacy can be reduced if the interval exceeds a month (30 days), dose on or about the same day of the month, give a late dose immediately and resume the schedule
• 05DailyMed: ProHeart 12 label12 months of prevention, not given more often than every 12 months, dogs 12 months of age or older
• 06DailyMed: ProHeart 6 labelEvery 6 months, not given more often than every 6 months, dogs 6 months of age or older
• 07FECAVA: Canine heartworm disease (2018, PDF)Prepared with ESCCAP and ESDA. Year-round prevention in southern Europe; in endemic areas of Central and Northern Europe, start within a month of the mosquito season beginning and continue until a month after it ends

Frequently asked questions

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