This is due to an over growth anaerobic bacteria.
The tongue is biggest harborer of this bacteria.
You need to buy hydrogen peroxide available at any store.
You should do the following routine daily.
1. Brush your teeth
2. After you are done brushing your teeth, brush your tongue.
Try to brush as far back as possible.
3. Take a cap full(about 1 teaspoon) of hydrogen peroxide and 1-2 to capfulls of water (try one first, if you can't tolerate it then add a second cap full). Now gargle with the solution for at least 2 minutes. This gets rid of the bacteria in the back of the mouth.
After that hold the mixture in your mouth for around 10 -15 minutes and swish around every now and then.
Do this in the morning and before you go to sleep.
Also cut down on sugar!
You should be cured. Tell me how this works.