Mountain Dew is made up of BOV, Brominated Vegetable Oil, carbonated water, high-fructose corn syrup (in much of the U.S.), concentrated orange juice, citric acid, natural flavors, sodium benzoate, caffeine, sodium citrate, erythorbic acid, gum arabic.
The complete ingredients for Mountain Dew though are a trademark secrete but we can know at least some of the ingredients to Mountain Dew.
Mountain Dew is banned in countries such as Europe and Japan.
The reason Mountain Dew is banned in both Europe and Japan is that the Mountain Dew contains brominated vegetable oil (BVO) which is a flame retardant chemical that is also banned in those countries.
If you want Mountain Dew or other sodas that contain the BVO or brominated vegetable oil then you'll be out of luck in Japan and Europe.
You could likely take Mountain Dew into Japan or Europe if you're traveling there but you may not be able to keep it.
It's stupid that the Mountain Dew is banned in those countries but it's banned for some reason that they see as a good thing for banning the Mountain Dew and other sodas containing the BVO.
Mountain Dew is also not the healthiest of drinks but okay to drink in moderation just like other sodas.