See comments below.
Image retrieved 7/25/2009 from http://blog.modernmechanix.com/2007/04/15/listerine-cures-everything/:

I tried to post a comment on the web page cited above,
but it didn't work because the comment was too long. So I created this
web page and posted a comment on the web page cited above with a link
to this page in the post.
=====
Never having seen any ads like this, a few years ago I independently started to gargle
twice a day with Cool Mint Listerine. I continued
because I went from many colds every winter to almost none, and the few
I do get are much much less severe. Additionally, when I start to feel
a tickle in my throat, I reach for the Listerine and gargle. The tickle
usually goes away.
So I generally believe in the results the ad claims. But my conjecture
is NOT that these results are primarily due to the antiseptic
properties of Listerine, but rather to the cleansing action in removing
accumulating mucous and entrained microbes from the throat and lower
nasal cavity (more thoughts below). Because of that, in an effort to improve the results, I
also started using saline nasal spray before I gargle with Listerine
(maybe I'll get a Neti pot, too). I've tried gargling with saline
solution but the results (clumps of mucous in the spit) are much less
favorable. There must be something in Listerine that cuts away or frees
the mucous from the throat tissues. Perhaps someone could recommend an
enhanced saline solution prepared at home that would do the same thing
as the Listerine. ?
Assuming what I've experienced and the ad claims is mostly true, why
doesn't the company currently manufacturing Listerine continue these
kinds of ads? Potential reasons:
1. After prolonged use there were some deleterious effects, but I have
never heard of any. Besides, using Listerine as only a mouth wash as
currently advertised would probably have the same deleterious effects.
So I don't think this is the reason.
2. The taste of the original Listerine was so bad nobody wanted to use
it anyway. Well, maybe, but in that time period people regularly
"enjoyed" Horehound Drops and Moxie Soda, so I don't think taste was a
deterrent to gargling with Listerine.
3. Gargling with Listerine is so effective that that use would
seriously and negatively impact the company's ability to market
multitudes of other cold and flu remedies that are much more profitable.
My money is on 3.
-Steve
P.S.: This past year especially, with the advent of "Swine Flu", health
organizations and the
media constantly recommend that we wash our hands frequently to
prevent the spread of flu and colds. Surgeons scrub their hands before
surgery. Doctors do the same before and after examining a
patient. See http://www.cdc.gov/Features/HandWashing/ and
http://www.who.int/gpsc/5may/en/.
These actions DO NOT kill any microbes, especially viruses. But the
washing DOES remove the microbes before they infect tissues.
This, I think, is the action of gargling with Listerine. It removes the
microbes before they have a chance to infect the tissues.
Also, a cold is caused by the infection of the tissues by a virus. But
many of the uncomfortable symptoms of a cold are caused not by the
virus, but by secondary infections of bacteria that are incubated in
the warm excess mucous the virus induces the tissues to excrete.
Gargling with Listerine removes the excess mucous and the microbes
living and reproducing in it, reducing or eliminating the symptoms caused by them.
Now that we're facing a real threat of losing thousands of lives to
"Swine Flu" this coming Fall and Winter (if attempts at making an
effective vaccine in sufficient quantities is unsuccessful), wouldn't
it make sense to hedge our bets by gargling with Listerine?
Further, why doesn't the US Center for Disease Control and the World
Health Organization do more research on how to prevent diseases along
these lines and develop more low cost and/or home made products like
Listerine? The
pharmaceutical industry has VERY LITTLE motive to do so because they
make much less money by PREVENTING or CURING diseases, they make much
more money
by TREATING diseases.