because its a decongestant, it get into the mucus and starts to break it up, and draws all of the water out of your body into your mucus. if you dont drink a lot of water, it severely dehydrates, you then your mucus will actually get harder to break up because now there is mucus and decongestant. all the mucinex does, go into the mucus and draws water into it. the water is actually what makes the mucinex work, and makes the mucus in your lungs easier to cough up. also lots of water helps get you better faster when your sick.