Your Dog Missed A Dose Of Heartworm Medicine…Now What?

Most monthly heartworm medications offer coverage for heartworm prevention beyond the typical 30 days.

A common question clients often ask is what they should do if they missed a dose of heartworm medicine: Should they have their pet tested first before giving another pill?  The answer to this question is no.  Fortunately, most monthly heartworm medications do offer coverage for heartworm prevention beyond the typical 30 days. The product Sentinel,  for example, is  often effective for 45 days, while Heartgard or Iverhart may be effective for up to 2 months; therefore, animal guardians can simply often resume giving the preventative at the appropriate time the next month.

If a pet misses more than one dose and up to 6 months, I will often instruct the client to simply restart the medication as well, in addition to instructing them to have their annual vet exam and heartworm testing at that time. It takes at least 6 months for a pet to become seropositive on blood tests after being bitten by a mosquito carrying heartworm larvae. Therefore, in clients missing more than 6 months of heartworm preventative medication, it is best to have the blood test first before resuming the administration of the medication.

Another common question asked is if a pet vomits after the administration of heartworm medication, I will typically tell clients to resume the monthly heartworm medication the following month. If vomiting occurs with subsequent administration, it may be best to change to a different brand of monthly heartworm medication or consider the topical medication Revolution, due to the possibility of having an allergy to the ingredients in the preventative.


  1. Nora Bannerman says

    The test of HW was positive for two of my dogs and I need two treatments. One of my dogs weights 58 lbs., and the another is 64 lbs.

    • Dr. Michael Dym, VMD veterinarian says

      I would recommend seeing local vet for possible heartworm adulticide treatment with injectable drug called immiticide.

  2. Suzanna McMahan says

    I have a for year old sweet Beagle/Rat Terrier Mix I adopted and he was HM + when I got him two years ago. These past 3 months have really taken a toll on me financially and was not able to get his Hartguard. Will he be okay? What can I do to ensure he will live for a long time? Thanks.

    • Dr. Michael Dym, VMD veterinarian says

      I would just feed him an all natural diet like wysong epigen diet from 1800petmeds, as well as get him heartworm tested and back on heartworm prevention. Consider multi vitamin like vita chews from 1800petmeds, as well as antioxidant like proanthozone for long term health.


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