Cola was a carbonated, sweet soft drink, most commonly in the form of the Coca-Cola brand, also referred to as Coke.
While on a an aeroplane to America, Jack Harkness requested a glass of cola. Lyn Peterfield spiked the drink with arsenic, poisoning him. (TV: Rendition)
Darius Pike drank cans of cola from the brand Best. (TV: Black Hunger)
The Eleventh Doctor ordered a Coke at Gloria's Golden Grill in Utah, and then used his fizzy drink straw to make it fizzier. (TV: The Impossible Astronaut)
The Twelfth Doctor explained the TARDIS' dimensionally transcendental nature to Maebh Arden by comparing it to a small glass of Coke having a large amount of sugar in it. (TV: In the Forest of the Night)
As a mixed drink Edit
Cola could be added to a spirit to create a mixed drink.
Polly drank several cocktails, drank a rum and Coke while at the Regent Hotel bar. (PROSE: Dying in the Sun)
Shou Yuing had a vodka and Coke while she talked to Ace. (TV: Battlefield)