It's Always Sunny in Philadelphia Wiki

Charlie and Mac promoting Fight Milk

Fight Milk was a drink created by Charlie in an effort to save Atwater Capital. Charlie believed the company needed a new product, and that the growing crow population in the city could be exploited. Hence, he worked with Mac to create "the first alcoholic, dairy-based protein drink for bodyguards by bodyguards." Charlie and Mac presented a promotion of the drink to Atwater Capital's investors, to no avail. ("Frank's Back in Business")​​

Little is shown of the drink in its earliest incarnation, but it appears to be contained in a jug with a logo of a muscular man with a crow's head, wrapped in one of the penis fliers from the ill-fated party. The drink itself is a mixture of crow egg, milk, and vodka, described by Mac and Charlie as being high in "crowtein." ("Frank's Back in Business")

Some time in between seasons 8 and 12, Frank's Fluids, LLC began producing Fight Milk to be sold. Fight Milk soon began gaining traction as a quick and easy way to expel fluids, being especially popular with UFC fighters. ("Wolf Cola: A Public Relations Nightmare")

Though excited by the new-found cash flow, Mac and Charlie ultimately felt more perturbed than pleased, feeling that their vision for Fight Milk--a growth-promoting drink for bodyguards, by bodyguards--was not being applied by its drinkers. As a result, the pair decided to reformulate the drink to better its efficacy as a bulking agent, while, due to popular demand, also creating a modified "gross" version of this new formula that maintains the role of fluid expellation (achieved by the addition of crow fecal matter). Thereby, the new duo of drinks now aided the strength trainer in both the bulking and cutting phases.

The results were a roaring success. However, all Fight Milk drinkers soon tested positive for performance enhancing drugs, at which point Mac and Charlie revealed that they pumped the new formula with human growth hormone (HGH).