I would never miss a month of medication under any circumstances!!!
What's a few bucks compared to the possibility of my losing my Springer Spaniel? I lost one to heartworms many years ago and I still suffer greatly over it, even though I know I could never suffer as much as she did in the end. Believe me in that you never want to see the end of that. You will never be the same.
I live in Florida but even if I lived elsewhere, I would never miss a month. I have also read that heartworm medication must be adminstered EVERY month in order for it to be effective. I don't know if that is true or just some hype to sell more of the stuff, but, I, for one, will take no chances.Â
I use Bayer which is an all in one. I get it from a place in Australia and time my orders so that delivery is free. They are friendly, helpful people.
http://www.pets-megastore.com/It's the least expensive place I have found. Make sure you put the hyphen between pets and megatore or you will get something entirely different.
It is true that fleas can develop an immunity to some medications, especiallly here in Florida, but I have't experienced that yet with Bayer. If I do, then I will switch.