Cola

 was a soft drink.

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)

As a mixed drink
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)