heartworm prevention for small dog

I use Heartguard....but talk to your vet.  You can't get it without an RX anyhow.

Heartguard's lowest dose is for dogs weighing between 1-20lbs. My question is whether that would be too much for her since she only weighs close to 2lbs.
 
I know I can only get these by Rx but I wanted to make an informed decision before I choose one over the other.

[linkhttp://www.fda.gov/cvm/FOI/1365.htm]http://www.fda.gov/cvm/FOI/1365.htm[/link]

I don't know if this makes a difference or what the real protocol is but I believe the cat doses are for a 1-3 pound cat or 1-20 pound dog.  I am not sure if those are the exact weights (cat vs. dog) but if you read the box it says.  Also the two species may matabolise differently.  But it's something to think about, perhaps quiz your vet about.  I saw that information on both the Heartguard and the Interceptor boxes.

If you live in an area that is a low risk area for heartworm (extreme north-colder regions) you may want to check into Revolution.
We don't reccomend it here in the south for dogs because the mosquito is the state bird in some instances.[]
 

My dog is 65lbs (golden/irish mix). I use Intercepter and a spot on flea product. I have also used Revolution (spot on flea and heartworm combo). Happy with both. As a note, we used the Revolution when we had an issue with mites and it worked great.
My girlfriend uses 1/2 of the intercepter, based on her vet's advice (her dog weighs 3 pounds).

My experience with Revolution wasn't good.  While on Revolution I HAD to also use Frontline to control the dreadful flea investation.

Glenda
I have never had good luck with Revolution on large dogs.  It looks like you are a big dog gal.  It works great on cats and small dogs though.

Well, gosh, to ME cockers are small!!  And my Misty was just as BAD as the shepherds.

Sorry Glenda
I saw the signature picture of the GS and thought you where a big dog person. 
I had a cocker years ago.  She was a lovely black girl.  I found her sitting on the side of the road.  She was in heat, hrtwm postitive, epileptic, and had cataracts.
She was the most wonderful dog.  I still miss her.

Not a problem!  I have six gsds now, but I've had cockers and other "smallish" dogs for many years.[]

My vet stopped carrying Revolution because several of his clients dogs came down with heartworms while on it.  I never used it, just the Interceptor and FrontLine for fleas & Ticks.

My girlfriend uses 1/2 of the intercepter, based on her vet's advice (her dog weighs 3 pounds).

The problem with this is, the pills are not scored, so they may not have the amount of medicine evenly distributed, you don't really know how much you are giving.

What area do you live in?  If it were me (depending on where you live) I'd do either the small interceptor every 45-60 days or don't do any prevention (IF you live in an area that has a low prevalence of HW) and test twice a year.  If she were to come up positive, you can give her a pill because it takes about 7-8 months for baby HWs (microflaria) to mature.  If you test her every 6 months, you should catch them in time to kill them before they mature.  I wouldn't risk this in a high HW prevalent area, though!
 
ETA: just re-read your post.  How old is she?  If she should get up to 5-6 pounds, and is younger than six months old now, I'd just wait until she gained weight before starting her.  It is safe to start HW prevention without testing before 7 months of age.  Or, once she reaches her adult weight, have her tested and then start the interceptor.

I use Sentinel and the lowest dose is for 2-10# dog.  I would go with the advice of your vet.