Understanding Heartworm Prevention
Heartworm is a parasitic disease transmitted by mosquitoes. When an infected mosquito bites your dog, it deposits microscopic larvae that migrate to the heart and lungs over 6-7 months, growing into worms up to 12 inches long. Left untreated, heartworm disease causes heart failure and death.
Monthly heartworm preventives work retroactively — they kill any larvae that entered your dog's body during the previous 30 days. This is why monthly timing matters: a gap of more than 30 days allows larvae to mature past the point where preventives are effective.
Treatment for an active heartworm infection costs $1,000-$3,000+, requires months of exercise restriction, and carries risks. Prevention costs $6-$18 per month and is nearly 100% effective when given on schedule.
Month-by-Month Prevention Schedule
Pick the same calendar date each month and administer the preventive on that date. Consistency is easier to maintain than counting exact 30-day intervals.
January
Year-round areas: maintain schedule. Seasonal areas: continue if active mosquito season.
February
Give on the same calendar date as January. Consistency is more important than exact 30-day intervals.
March
Seasonal areas: mosquitoes may begin emerging. If you paused, your vet may recommend a heartworm test before restarting.
April
Spring mosquito season begins in most of the US. All dogs should be on active prevention by now.
May
Peak mosquito breeding begins. Double-check that your household knows who is responsible for the monthly dose.
June
Summer peak. If traveling with your dog, maintain the schedule regardless of location changes.
July
Mid-summer. Many owners miss doses during vacation months — set a dedicated tracker alert.
August
Continue monthly dose. If you adopted a new dog, get a heartworm test and start prevention immediately.
September
Mosquito season remains active in most regions. Do not stop early.
October
Fall transition. Mosquitoes can survive until the first hard frost — keep dosing.
November
Seasonal areas: some vets say you can stop. The AHS recommends continuing year-round to eliminate gaps.
December
Year-round dosing continues. If you use a monthly chewable, this is a good time to refill for the next year.
What to Do If You Miss a Dose
Missed by a few days
Give the dose immediately and continue on your regular monthly date. No vet visit needed for a brief gap.
Missed one full month
Give the dose immediately. Monitor your dog for signs of lethargy or coughing over the next few months. Consider a heartworm test at your next vet visit.
Missed 2 or more months
Contact your vet before resuming. They will likely recommend a heartworm antigen test. Giving preventive to an already-infected dog can cause a serious inflammatory reaction.
Dog was never on prevention
Schedule a heartworm test before starting. If the test is negative, your vet will prescribe a monthly preventive that day. Test again 6 months later to confirm.
How a Medication Tracker Prevents Missed Doses
Monthly medications are the hardest to remember consistently. Unlike daily medications that become part of a routine, a once-a-month dose is easy to forget — especially during busy months, vacations, or schedule changes.
Persistent monthly reminders
Alerts that escalate until someone confirms the dose was given — not just a dismissible notification.
Household coordination
Every family member sees the same schedule and knows whether this month's dose has been given.
Adherence history
A 12-month log showing every dose date, making annual vet discussions and refill timing simple.
Multi-pet support
Separate profiles for each dog with individual heartworm schedules, especially important when dogs are on different products or doses.
Frequently Asked Questions
What happens if I miss a month of heartworm prevention?
If you miss one monthly dose, give the next dose as soon as you remember and continue on the normal monthly schedule. Because heartworm preventives work retroactively (killing larvae from the prior 30 days), a single missed dose can leave your dog exposed. If more than 2 months are missed, your vet may recommend a heartworm test before resuming prevention, since giving preventive to an infected dog can cause serious reactions.
Should heartworm prevention be given year-round?
The American Heartworm Society recommends year-round prevention regardless of where you live. Mosquito seasons are unpredictable, and a single lapse allows risk of infection. Year-round dosing is also easier to maintain as a habit — stopping and restarting seasonally creates opportunities for missed doses and gaps in protection.
At what age should puppies start heartworm prevention?
Most heartworm preventives can be started as early as 6-8 weeks of age, depending on the product. Your vet will recommend a specific product based on your puppy's weight and breed. Do not wait until the puppy is older — early protection is important since heartworm infection can occur at any age.
How much does heartworm prevention cost per month?
Monthly heartworm preventives typically cost $6-$18 per month depending on the product, your dog's weight, and where you purchase it. Injectable options like ProHeart 12 cost more upfront ($50-$150 at the vet) but provide 12 months of protection in a single dose. Compared to heartworm treatment ($1,000-$3,000+), prevention is far more affordable.
Can I use a phone app to track heartworm medication?
Yes, and it is one of the most effective ways to avoid missed doses. A dedicated pet medication tracker like PetTimely sends persistent monthly reminders and confirms when the dose is actually given — not just when the alarm went off. It also tracks which family member administered the dose, preventing the 'I thought you gave it' problem.
Never miss a heartworm dose again
PetTimely launches on iOS in Q2 2026 with persistent monthly reminders, family coordination, and complete medication adherence tracking for heartworm prevention and every other medication your dog takes.
Join the PetTimely WaitlistDisclaimer: PetTimely is an organization and tracking tool. It does not replace a licensed veterinarian. Always consult your vet about heartworm prevention, testing schedules, and treatment options for your dog.