I've always kept bottles of fizzy drinks inside the fridge, but I've been told by many that doing so will result in the drink becoming flat much quicker and that I should keep bottled fizzy drinks outside the fridge. I guess I've just blindly accepted the "keep refrigerated" marker on the bottle. So, the question is, which is correct? Should they be kept outside the fridge or inside?
Solubility of gases, including CO2 decreases with temperature.
Thus, with constant pressure, warm carbonated drink will go "flat" much sooner than cooled one.
In case of sealed bottle, effect is much lesser, because solubility increases with pressure. Leaving your sealed soda bottle warm will build up pressure with CO2 escaping the water, preventing more gas from escaping the solution. This is described by Henry's law.