Aspirin

Aspirin was a drug used by humans in tablet form to relieve mild pain.

Aspirin and Time Lords
Aspirin was a poison fatal to Time Lords.

Jo Grant once offered an unidentified tablet to the Third Doctor, who declined because it was for the wrong metabolism and would probably kill him. (TV: The Mind of Evil)

The Sixth Doctor found himself poisoned by a large, concentrated dose of aspirin, which he knew to cause severe allergic, pulmonary and cerebral embolism in Time Lords. He fell into a deep coma while his body fought off the effects. (PROSE: Burning Heart)

Another such attempt was made on the Sixth Doctor's life when an alien who knew Time Lord physiology poisoned his tea with soluble aspirin. The Doctor counteracted the aspirin by eating chocolate, a source of simple triglycerides which the Doctor's body easily absorbed. This negated the anti-platelet effect of the aspirin. (AUDIO: The Condemned)

The Seventh Doctor knew one good dose of aspirin would be enough to kill him. (PROSE: The Left-Handed Hummingbird)

One of the Time Lords from The War's future amused himself by taking powdered aspirin as a, which would have killed a normal Time Lord in seconds by interfering with hormone receptor intermediaries. (PROSE: The Taking of Planet 5)

The Eighth Doctor, frustrated by the lack of service he found in a local government building, claimed he wanted to take an aspirin for the headache he'd gotten from the situation. (AUDIO: Caerdroia)

Other appearances
In 1555, Francois de Noailles accused Evelyn Smythe of plotting to use the aspirin she brought with her to poison Queen Mary I. (AUDIO: The Marian Conspiracy)

In 1979, Professor Chronotis offered Chris Parsons some aspirin for his headache. (HOMEVID: Shada, WC: Shada)

By the 25th century, aspirin was no longer used. Barbara Wright got some aspirin from the TARDIS' first aid kit for Vicki's headache, but she compared taking the pills to the use of leeches. (TV: The Web Planet) Later, the Doctor no longer carried aspirin in his TARDIS. (PROSE: The Infinity Race)

Behind the scenes

 * The tablet which would have killed the Doctor in The Mind of Evil was not specified as aspirin, but was assumed to be so. It became fanon after Kate Orman specified the Doctor's weakness toward aspirin in The Left-Handed Hummingbird. Other references followed that novel.