This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
The monthly medications are Heartgard and Interceptor.
Heartgard is Ivermectin and Interceptor is Milbemycin Oxime.
These medications work by killing any larvae that have entered
the dog up to 45 days ago. They kill L3s, 4s, and 5s. These
drugs are given monthly (30 days) for the convenience of giving
on the same day each month and also to give you a safety
margin. If you forget to give your dog his/her heartworm
medication, you have about 15 days to remember to give it and
the dog will still be protected. With the daily medication,
forgetting for more than a day may result in your dog becoming
Most common ways that a dog will contract heartworms while on
medication include not being given medication on a regular
basis (e.g. completely missed dosages); traveling from a winter
environment to a summer environment like Florida without giving
the dog heartworm medication; not WEIGHING the dog while on the
medication: the dog outgrows its dosage; and the dog vomiting
or having severe diarrhea after being given its medication.
What should you do if you forget your dog's medication? *IF*
the dog is on daily medication, give the dog a monthly tablet
within 45 days of the missed dose. Depending on what you feel
comfortable with, you can then restart the dog on the daily
medication, or continue giving the medication once a month.
*IF* your dog is on monthly medication, give the medication
anytime you remember, even if more than 45 days has passed.
Giving heartgard to a dog with heartworms will not hurt the
dog, and until 6 months has passed the dog will appear to be
negative anyway. However, you should NEVER give daily
medication to dogs who may have circulating microfilariae. The
daily medication can cause an anaphylactic reaction if given to
a dog with microfilariae present. Giving monthly medication
will prevent the dog from acquiring a heavy worm load by being
bitten by multiple infected mosquitoes. Just be certain to have
the dog tested 6 months after the missed dose to be sure that
the dog did not acquire heartworms.