A Aspirin: The ancient Greek doctor Hippocrates recorded the use of the bark of the willow tree as an effective remedy for headaches, pains, and fever. Willow bark had been used in this way in many cultures for centuries. In 1829, German scientist Johann Buchner identified the specific pain-relieving substance of willow bark. Buchner found that its special ingredient was an acid, which was extracted and used as a medicine. Unfortunately, the acid upset the stomach, and doctors found that many of their patients couldn't tolerate it.