Before modern Mountain Dew soda was invented, primitive Siberians brewed lemon and lime juice with bone broth, rainwater (leading to the term Mountain Dew, as this water was often gathered from melted snow on mountains during the dry season) and calcium crystals, and let the resulting mixture ferment for absurdly long amounts of time (some brews from over 500 years ago remain fermenting to this day). The resulting drink would be incredibly potent and chalky, and would sate thirsty Siberian travelers for days due to the illusion created that a portion of the drink large enough to last days had been consumed when the consumer had only taken a small sip or even a whiff of the brew.
After the modernization of the beverage into what we now know as the popular soft drink Mountain Dew, the fermentation process shortened to a few months before the desired potency was achieved. The drink became outlawed in Siberia and most of the world after the fermentation process became widespread and the drink began causing problems for those who weren't genetically disposed to handle the beverage, unlike the Siberians.
Although no videos exist of the drink's consumption prior to Dimitri's review, his potent reactions paint a strong picture of the drink's effects. The drink is historically known to satisfy hunger and thirst through its hallucinogenic properties, as well as to strongly energize the consumer for harsh travels. In Dimitri's case, as he is especially fragile, the drink overwhelms him and causes him to spray it all over the floor of the abandoned home he finds it in. (This may also be due to the fact that it is unknown how long the Expired Mountain Dew had been fermenting, meaning it could possibly be much stronger than usual.) It also causes him to gasp and cough, possibly due to his respiratory system being under stress from his increased heart rate, and have difficulty pronouncing words, including the word "sweet," which he pronounces as "suhwEEEEEHHHHHHHHHHH." Afterwards, Dimitri comments that the Mountain Dew tasted chalky (most likely due to the calcium crystals), as well as fermented and incredibly sweet (the typical result of the fermentation process). The liquid may have also been highly corrosive (the fermentation process can vary in effects), so it was quickly cleaned off the floor of the home.