American Heartworm Society | FAQs
From the above link:
How do monthly heartworm preventives work?
ANSWER Whether the prevention you choose is given orally, topically, or by injection, all approved heartworm medications work by eliminating the immature (larval) stages of this parasite. This includes the infective heartworm larvae deposited by the mosquito as well as the molted fourth larval stage which develops inside the animal.
Unfortunately, within 2 months, immature heartworm larvae can molt into an adult (fifth) stage, which cannot be effectively eliminated by preventives. Because heartworms must be eliminated before they reach this adult stage, it is extremely important that heartworm preventives be administered strictly on schedule (monthly for oral and topical products and every 6 months for the injectable).
Administering prevention late can allow immature larvae to molt into the adult stage, which is poorly prevented.
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There are always people who will believe the Jim Jones type folks. I personally do not care for purple kool aid.