Salicylic acid, derived from willow bark, was a potent painkiller but often caused severe bleeding ulcers. Bayer added an acetyl group to create a much gentler compound.

Prostaglandins are lipid compounds that trigger inflammation and fever when tissue is damaged. Aspirin relieves pain by inhibiting the COX enzymes needed to synthesize them.

Chewing the tablet speeds up drug absorption into the bloodstream. The aspirin rapidly prevents blood platelets from sticking together and enlarging the dangerous clot.

Aspirin is formally known as acetylsalicylic acid. When exposed to humidity, it breaks down into salicylic acid and acetic acid, the same compound that gives vinegar its smell.