doodle is currently taking Simparica TRIO (every 6 weeks year round). I chose this brand specifically due to the fact that I live in the South and NC, USA is home to fleas, ticks and mosquitos. Our swampy wetlands may be popular with duck hunters, but even duck hunters love their dogs!
I do not give her the meds every month because I suspect that manufacturers tend to put in more meds than really necessary. It sounds counter intuitive, but in practical terms: why would you give a 44# dog the same dose as a 22# dog? So, since doodle isn't 44#s, I add a couple weeks to the dosage to prevent a chemical build up in her system. She doesn't seem to display any noticeable (scratching) issues until the day or two before her next dose.
This diagram illustrates that heartworm treatment is effective between 1-2 months after the related mosquito bite transmission. That information was relevant in my dosage theory. Because there is a 4-8 week window before the typical heartworm treatment becomes ineffective, So... I question the need to medicate our pets every single month (and I want to think I'm doing it right).
Naturally, you should make your own decision, based on where you live, the prevalence of mosquitos, fleas, and ticks, and how your dog reacts to the treatment you have chosen. doodle has been taking Simparica TRIO 22.1-44.0# tablets for a year and she seems to be doing well with it.
climbing off soapbox