Advantage Multi gives flea protection AND heartworm prevention, for about $14/month if ordered with your vet's RX from KV Supply. It's a cost-effective "all in one." It's very effective in my region, where we need year-round flea and HW protection.
It's labeled to start at 7 weeks, I think.
The latest I know of a pup starting and staying negative in my area is 16 weeks, and I think he only stayed negative because he was on Advantage Multi (not Heartguard), since that's the only product with evidence that it actually kills juvenile and young adult worms (slowly).
The longer you wait, the higher the risk. I don't want to delay and end up with a young dog with baby heartworms growing in it -- which has happened to many people here...and then you're looking at about $1000 of HW treatment. In the Gulf Coast region, if you delay starting HW prevention until a dog is 6 months old, you probably will already have a dog with heartworms by the time it gets its first dose of prevention. It just won't test positive for another 6 months or so--but they're already in there, growing.
If you want to separate flea and HW, make sure you are not in an area of ivermectin resistance (= Mississippi Valley and Delta). It still works very well in the vast majority of places, though. Ivermectin is the ingredient in Heartguard.
If you are in an area with tick disease, you also have to factor that in, as it can be extremely important because some tick-borne diseases are very serious and hard to treat (Advantage Multi does not work against ticks).
Frontline Plus works well in some areas and not at all in others. In my area, the fleas appear to have possibly developed some resistance to fipronil (the ingredient in Frontline Plus and Pet Armor)--at least, that's what shelter vets tell me. The newer generation of vet RX products seem to be extremely effective against both fleas and ticks (e.g, NexGuard).
Last edited by Magwart; 09-18-2016 at 01:26 PM.