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Ninety percent of the population experiences at least one headache each year. The most common type is a tension headache, which is caused by stress and is characterized by tightening of the muscles in the base of the neck and along the scalp. Aspirin alleviates headaches by blocking the body's production of prostaglandins, hormones that contribute to pain by stimulating muscle contraction and blood vessel dilation. For thousands of years, people chewed the bark of willow trees to control headache and other pain. The study of the properties of this medicinal plant led German chemist Hermann Kolbe to synthesize acetylsalicylic acid (ASA), a building block of aspirin, in 1859. A pure form of ASA wasn't prepared until 1897, by Felix Hoffman, a chemist in the Bayer chemical factory in Germany. After publication of successful clinical trials, aspirin was distributed in powder form in 1899 and as a tablet in 1900. Aspirin possesses a number of properties that make it one of the most recommended drugs. Besides being an analgesic, or pain reliever, it also reduces inflammation that often accompanies injuries or diseases, such as arthritis. It is also an antipyretic compound, or fever reducer. Aspirin is the only over-the-counter analgesic approved for prevention of cardiovascular disease. New research suggests that aspirin may also decrease the risk of some forms of stroke. Additional studies indicate that aspirin may play a role in reducing the risks of ovarian cancer.