Milk

Milk was a liquid secreted by mammals to feed their young. Different species could usually consume the milk of other species, as humans and Time Lords commonly did with the milk of cows. (TV: Hide) It was often used as an accompaniment to tea. (PROSE: Fungus) The Ninth Doctor once implied that there was something unique in the universe about the milk that came from cattle, and that he regularly had to return to Earth just to get milk. (TV: The Empty Child) The Tenth Doctor later told Martha that he normally went to a farm in 9th century France for his milk. (COMIC: Wrath of the Warrior)

Milk-producing species
Whether the female of a species exuded milk from its nipples or from simple ducts to nurture its young was one of the defining traits of mammals. (PROSE: The Year of Intelligent Tigers) Similar nourishing substances were produced by non-mammalian species. Malmooth could drink their own internal milk, (TV: Utopia) while the green milk produced by Skarasens was fed on by the Zygons. (PROSE: Sting of the Zygons, PROSE: The Bodysnatchers)

The only known cure for spectrox toxaemia was the milk from a queen bat a species native to Androzani Minor. The cure was discovered by Professor Jackij. (TV:The Caves of Androzani)

The seemingly male Sontarans could be genespliced to produce milk. Strax was very proud that he could produce "magnificent quantities" of lactic fluid and offered to nurse Melody Pond. (TV: A Good Man Goes to War)

Milk as a beverage
Milk was also used by other species as a drink. The humans of Earth harvested and drank the milk of cows. Milk was sold in cartons or glass bottles, and delivered by milkmen. (TV: The Stolen Earth, Hide) In some locations, such as the planet Sto, milk was sold in a Milk Market. (TV: Voyage of the Damned)

Milk was often used in tea and coffee to add taste. It was sometimes drunk warm as a sleep aid. (PROSE: Theatre of War, TV: Invasion of the Dinosaurs)

Like humans, cats often drank milk. One one occasion, this made the cats of the United Kingdom vulnerable to alien attack. When a fungus from space found its way into Britain's milk supply, cats all over the country turned on their owners in a series of savage attacks. (PROSE: Fungus)

Milkshakes were served in Iceworld's ice cream parlour. The owner Anderson reassured a customer they didn't stain after Ace poured some over the head of the customer. She then poured another over Anderson's head too.

The Doctor
The Fifth Doctor, with the use of a half empty tank of oxygen, had to climb into the caves on Androzani Minor to obtain some bat's milk to cure himself and Peri. The milk was a cure for spectrox toxaemia. Sadly, the Doctor only managed to milk enough for Peri. As Peri came round the Doctor was in the late stages of the disease and he regenerated into his sixth incarnation to cure himself. (TV: The Caves of Androzani)

The Seventh Doctor, in 1963 brought freshly delivered milk to Martin when he visited the Hand of Omega at the Funeral Directors. (Remembrance of the Daleks).

The Ninth Doctor travelled to Earth at least every five days, which Rose Tyler attributed to him having to get milk. He seemed annoyed that, of all the universe, it had to come from a cow. (TV: The Empty Child)

The Tenth Doctor and Martha Jones fought and then helped a viking warrior they ran into when they stopped to buy milk. (COMIC: Wrath of the Warrior) Another time they tried to get milk they ended up in the Whispering Gallery instead of a grocers as intended. (COMIC: The Whispering Gallery)

Larry Nightingale met the Tenth Doctor when he returned from a milk run. (TV: Blink)

Rory Williams ran into trouble when he went to the market to get milk and a whisk for the Eleventh Doctor who was making proper ice cream in the TARDIS. He ended up pretending to be the Doctor in order to get an alien to leave. (COMIC: Rory's Story)

The Eleventh Doctor admonished Melody Pond, telling her that she should call Amy "Mummy", not "Big milk thing". (TV: A Good Man Goes to War)

After fighting several groups of monsters, the Eleventh Doctor met River Song and she asked him if he remembered to pick up milk. He dejectedly left again to get River's milk. (COMIC: Take a Bow (Tie))

The Eleventh Doctor once drank milk from a bottle. (TV: Hide)

Milk as an ingredient
Milk was also the base ingredient for many foods, known as dairy foods. The Tenth Doctor showed a great fondness for most foodstuffs made from milk, keeping track of which galaxies had the best milkshakes and most expensive ice cream. (COMIC: Agent Provocateur, Escape to Penhaxico)

The Eleventh Doctor claimed to have a doctorate in cheese making. (TV: The God Complex)

Shortly after regeneration, the Eleventh Doctor tried several foods, among them buttered bread, and yoghurt before settling on custard. (TV: The Eleventh Hour)