When I tried Mucinex in the past, I wasn't all that impressed with its abilities. It seemed to make me cough more and it didn't really make anything better. I've been using it again with this bout of the flu that I got, and it does actually seem to help. I think if I took it more religiously, I'd be more impressed with its capabilities.
Mucinex pretty much goes into your system and loosens the phlegm in your lungs so it makes your coughing more productive, in case you haven't heard of it or seen the gross little commercials they put out for it :) You can take it twice a day, and it really does do what it says it will. The problem I have with it is that it works a little TOO effectively! When I use it, my chest hurts from coughing so much! Yes, it loosens the curd, and yes it does make the coughing more productive, but with the extra coughing it does tend to hurt.
I still recommend it because it really does help and I think it probably lessens the "sick time" by getting all that stuff out of your system. When you use it, however, be prepared to cough a lot and be prepared for it to be a little gross!
Each tablet contains 600 mg of Guaifenesin in a bi-layer so it can give you a dose immeadiately and have some for extended release, too. You shouldn't use it in children under 12.
It doesn't seem to have any real side effects to me, but I encourage you to read the label to see if there are any allergy contraindications or side effect infromation you should be aware of before using this product. I've never had a problem personally.