Whisky

Whisky — sometimes whiskey — was an alcoholic drink served on Earth. Clara Oswald called it "the eleventh most disgusting thing ever invented". (TV: Hide)

Ajax Saloon served watered-down whisky. Unknown to the townspeople of Ajax Camp, the water was controlled by an alien from Saturn. Those who drank the whisky became possessed. (PROSE: Whiskey and Water)

The Third Doctor drank what appeared to be a whisky and soda whilst in Hart's office. (TV: The Sea Devils)

The Eighth Doctor once drank whisky. (PROSE: Sometime Never...)

Clara Oswald and Emma Grayling drank whisky at Caliburn House in 1974. Neither of them liked it, and they went for tea instead. (TV: Hide)

Jason Kane ordered a scotch at the Witch and Whirlwind. (PROSE: Deadfall)

Whisky in popular culture
In the television series EastEnders, a 2007 storyline had Peggy Mitchell being confronted by the ghost of Den Watts. She told him to get out of her pub, saying that the only spirits she served were gin, whisky and vodka. (TV: Army of Ghosts)

A song named "The Wild Rover" included the lyrics, "And I spent all my money on whisky and beer." (TV: Turn Left)

Behind the scenes
In the real world, whisky is the predominant spelling in Canada and the United Kingdom, while whiskey is used in Ireland, the United States and more or less globally. Both spellings are used in DWU texts, but the reason for the ambiguity of spelling does not appear to have been remarked upon in any story.

Indeed, very often the same writer will use different spellings in different books. Does Mark Gatiss mean to suggest that Edmund Trevithick drinks Irish whiskey in Nightshade, while Benny drinks Scotch whisky in St Anthony's Fire? It's hard to know for certain, but one interpretation of the presence in both spellings in DWU fiction is that the spelling gives us a subtle bit of genuine characterisation.

Of course, another interpretation is simply that it was the Target Books house style, or individual editor preference in later ranges, to use whisky, and thus we're seeing a false bias towards the British spelling.

In any case, whisky is the dominant spelling in books, by a margin of around 5:1.