In short, don't waste your money.
In long, really, don't, and here's why:
I've been getting cold sores since I was a kid; because, like 80% of all human beings, I carry the oral variant (HSV-1) of the herpes simplex virus (unlike the OTHER type of Herpes, HSV-2, which only affects about 25% of all humans. Yes, 25% of people have that kind of herpes, likely including people you've slept with).
Like I said, 80% of you are right there with me (though only half of you get cold sores regularly).
They call them cold sores, or fever blisters; because when your immune system is weakened by a cold or other illness, the virus which causes them is no longer suppressed, and starts reproducing like mad. This causes orofacial blistering; and in some cases oral ulcers of the tongue, soft palette, and cheeks.
If you get them,you know how much they suck. They itch, they're painfully sore, they look hideous, and they can spread from one little dot, to all over your mouth (technically to any soft tissue, but in most people it's limited to their mouth and lips; which is a very virus friendly environment) within an hour or two; swelling your lips up to Pam Anderson proportions.
There are several ways of treating them.
Most people just let them run their course, which is fine. They generally go away in a few days, to a few weeks at most.
There is actually a quite effective medication for them, sold over the counter in some countries under the brand name Zovirax; but in the U.S. it's only available by prescription (as Aciclovir). It's also rather expensive, and a pain to go and get a prescription for a simple cold sore.
There is another very effective treatment, that works, stops them from spreading, and guarantees you'll heal faster than if you just leave them alone. Lance the sores, and then flush them with alcohol repeatedly until they stop weeping.
Seriously, that's been the doctor approved method of dealing with severe cold sores forever. The alcohol denatures the protein coating of the virii, preventing them from reproducing; as well as sanitizing the skin, and now open sore, to prevent secondary infection.
Unfortunately, doing so is painful, itchy, looks almost as bad as letting the sores go on their own, ALSO causes your lips to puff up, and can lead to dermatitis, and even permanent scarring (most of the time it's not a problem, but I do have some slight scarring from a particularly bad flareup I had as a kid).
So, as you know, I am at the moment recovering from an immunosuppressive fungal infection, complicated by a secondary infection by the flu. Basically, cold sores were going to be a given; and they showed up right on schedule.
A few years ago, a third option for treating the sores became available; a topical cream with a 10% solution of Docosanol (a fatty alcohol), sold under the name Abreva here in the U.S.
Up until now I hadn't bothered trying it because of the cost (about $13); but this is the most severe flareup of cold sores I've had in years. Seriously, my mouth is totally fishlipped and swollen; and I've got an ulcerated tongue, making eating, drinking, and talking, all painful.
So I thought, what the hell, if it works, the $13 is a bargain; and instead of my usual lance and flush treatment (which leaves me at most with slightly swollen lips and a scab for a few days)I used the Abreva.
I really wish I hadn't.
The idea behind the Abreva cream, is that the fatty alcohol in a topical solution will be absorbed through the skin without lancing the sores, then do the same thing as flushing them with alcohol; only slower, and with less skin damage.
I knew the cold sores were going to come, and knew I needed to start abreva "at the first tingle", so I had Mel pick some up for me in advance. I started spreading it on the tingly area from the first tingel, then the other areas as they got tingly too; and it not only didn't slow down, or stop the blisters from forming; I'm pretty sure the abreva accelerated it.
Maybe that idea works for other people, but in my case, all the abreva did was numb my lips a bit, and make it easier for the sores to spread by drying out the skin at the edges of the sores (dry skin cracks, the virus laden fluid seeps into the cracks, then forms another sore).
My advice is, don't waste your money. If you need to stop the spread of cold sores, or need them to heal faster; either do the alcohol treatment as I describe, or pay for the Aciclovir prescription, because Abreva is just a waste of money.