Interceptor is supposed to be given once a month (I do 45 days) and it's a chewable tablet. The length of time between doses is based on the amount of time it can take a microfilaria to mature to the 'next step' where you want to stop that maturation from happening.
Sentinel can work fine if your dogs stay 'home'. It's Interceptor and the old drug "Program" (which has been around for a long time and isn't problematic usually -- ANY drug can be but it's pretty safe).
The argument that the HeartGuard people have put forth for years is that in small enough doses ivermectin is 'safe' for all breeds. The problem is that at this point they have reduced the dose SO much (so it is "acceptable" for all) that it's not much good at heartworm prevention, and it certainly doesn't do any good for other parasites.
However, I'll say this much and shut up ... at one time a LONG time ago, when we were very broke, my old vet told me I could use ivermectin and he helped me 'dose' it for all 3 of my dogs (a cocker, a boxer/pit mix already taking liquid ivermectin as heartworm 'treatment' because her heartworm was SO advanced the immiticide wasn't safe for her) and my old (then 10) sheltie/corgi mix.
I carefully dosed it for everbuddy -- this was like 10 years ago when they pretty well thot it WAS *only* collies that were problematic. And honestly I didn't even know that way back then. However, I noticed Foxy just plain came up missing that evening -- he hid in the dark back bathroom behind the toilet and we couldn't get him out for love nor kibble. Finally I realized he was really not feeling well -- light hurt his eyes, sound hurt him, and generally he wanted to be ALONE -- and for all the world, he looked like he had a MASSIVE headache.
I called Doc and he told me it sounded like Foxy was having a reaction to ivermectin ... that they did sometimes but not often (Doc was part of the original team that did research and development on ivermectin so he knew a LOT about it). He advised me to take that dose I'd given this time, and cut it in to 25% of what I'd used (one quarter of the dose) for next month. That it should protect him and be safe at that dose.
Nope. SAME symptoms only worse. In fact, I thought I'd lose him that night and literally I was way way below 'dose'. I was guilt-ridden -- I couldn't believe I'd caused him so much pain. Thankfully he had no lasting problems.
Ivermectin crosses the semi-permeable brain membrane of the parasite's brain and scrambles the neural responses in the brain so totally the parasite dies. THAT is how it 'kills' the parasite and not the dog.
EXCEPT, in herding dogs.
In herding dogs the ivermectin actually crosses the semi-permeable brain membrane of the DOG and begins to do, in the dog's brain, exactly what it would do to the parasite. (paraphrased from "Pill Book Guide for Medication for your Dog and Cat" - Dell Ref. 199
So yes, the dog feels like it's got the grandaddy of all migraines and worse.
AT that point Interceptor had just come out and was experimental -- but he ordered some and said hopefully it was better but to watch him. No reaction.
Accordng to my vet a dog can actually become MORE sensitive to ivermectin as time passes. It's not that it 'lasts' but rather it actually may do some damage that doesn't heal. That part was just Doc's speculation, but his stand was not to use it at all.
Given that dogs can't tell us if they have a mild headache, I'm not going to risk it.
I can tell you that my holistic vet, who teaches all over the country, found herself infected with Sarcoptic Mange after treating a feral pup at her clinic and halfway across the country on an airplane she discovered she was doing the "itchy scratchy" thing herself and was infected with sarcops (another blood-feasting parasite that ivermectin treats). So she took ivermectin HERSELF simply because she was on the way to a teaching engagement and had to treat it *now*.
She said to me one day "I actually know how the dog feels. You don't feel 'bad' nor really 'good' -- kinda just 'blah' -- and you feel like you have "bad breath" all thru your whole body. It passes after about 24 hours when it leaves your body."
But that was a reaction in a being not having a 'bad' reaction -- just the general body response to the drug.
At this point I only buy Interceptor even tho I don't have herding dogs. At one time my yard was exposed to whips and hooks and they ARE in my soil, so it's worth it for me to use the Interceptor and reduce the risk of those. Ivermectin is NOT a bad drug -- only on herders and a few other breeds. But I really have a problem with the company trying to make everyone believe it is SO safe, when likely reactive dogs feel like crap even tho it may not technically do them severe harm. After seeing Foxy's reaction never ever would I expose a herder to ivermectin under ANY circumstances.