According to the INTERNET:
“Hippocrates, the Greek physician who lived from about 460 to 377 B.C., wrote that willow leaves and bark relieved pain and fevers.
It wasn’t until thousands of years later that people began to isolate the key ingredients of aspirin. An 18th-century clergyman, Edward Stone, rediscovered aspirin, in effect, when he wrote a report about how a preparation of powdered willow bark seemed to benefit 50 patients with ague and other maladies, Roueché wrote.
In the 1800s, researchers across Europe explored salicylic acid. French pharmacist Henri Leroux isolated it in 1829, Roueché writes. Hermann Kolbe discovered synthetic salicylic acid in 1874, but when administered often in large doses, patients experienced nausea and vomiting, and some even went into a coma. A buffer was needed to ease the effects of this acid on the stomach.
The aspirin we know came into being in the late 1890s in the form of acetylsalicylic acid when chemist Felix Hoffmann at Bayer in Germany used it to alleviate his father’s rheumatism, a timeline from Bayer says. Beginning in 1899, Bayer distributed a powder with this ingredient to physicians to give to patients. The drug became a hit and, in 1915, it was sold as over-the-counter tablets”
Ironically the salicylic acid in willow bark is already buffered by other phytochemicals in the willow bark.