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