Aspirin

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

Aspirin and Time Lords

Aspirin is apparently a fatal poison to Time Lords, as their metabolism was different.

Jo Grant once offered a tablet to the Third Doctor, who declined saying that it'd probably kill him. (DW: The Mind of Evil)

An attempt was made on the Sixth Doctor's life when an alien who knew Time Lord physiognomy poisoned his tea with soluble aspirin. The Doctor managed to counteract the aspirin by eating chocolate, a source of simple triglycerides which the Doctor's body easily absorbed, and which counteracted the anti-platelet effect of the aspirin. (BFA: The Condemned)

Other appearances

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

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

By the 25th century, aspirin was no longer used. Barbara Wright got some aspirin from the TARDIS's first aid kit for Vicki's headache. (DW: The Web Planet) Later, the Doctor no longer carried aspirin in his TARDIS. (EDA: The Infinity Race)