What cause heavy mucus in the back of the throat?

hello, I was suffering from mucus to the point where I seriously became suicidal, It was winter and it was thick so I literally went to Thailand to change climate because i had been there several times, anyway when i was there they had this medicine called N-Acetyl-L-Cysteine in many different forms right behind the counter. It was sold to smokers(everyone in Asia smokes cigarettes) to relieve phlygm and this saved my life. I have no clue why it is not popular here. I smoked pot alot and had quit cause I was suffering and now am back to it daily:) in the states this drug is considered a vitamin and has been shown to boost your immune system and has shown signs of counter attacking bi-polar depression as well. it really saved my life, i was on here with no clear ansewr before i left and now i smoke daily and work in lawncare which would have been unheard of previously. you can research it on Wikipedia. the recommended dosage is 1000 milligrams daily, you might try that at first but i usually only take 500 mgs a day to save money. its cheap tho $8 for 100 capsules. ive bought it here before http://www.swansonvitamins.com/JR031/ItemDetail but it can be found many different places online. if you have any questions let me know and good luck:)