Aspirin, also known as acetylsalicylic acid (ASA), is a medication used to reduce pain, fever, or inflammation. Specific inflammatory conditions which aspirin is used to treat include Kawasaki disease, pericarditis, and rheumatic fever.Aspirin is also used long-term to help prevent further heart attacks, ischaemic strokes, and blood clots in people at high risk. For pain or fever, effects typically begin within 30 minutes. Aspirin is a nonsteroidal anti-inflammatory drug (NSAID) and works similarly to other NSAIDs but also suppresses the normal functioning of platelets.One common adverse effect is an upset stomach. More significant side effects include stomach ulcers, stomach bleeding, and worsening asthma. Bleeding risk is greater among those who are older, drink alcohol, take other NSAIDs, or are on other blood thinners. Aspirin is not recommended in the last part of pregnancy. It is not generally recommended in children with infections because of the risk of Reye syndrome. High doses may result in ringing in the ears.A precursor to aspirin found in leaves from the willow tree (genus Salix) has been used for its health effects for at least 2,400 years. In 1853, chemist Charles Frdric Gerhardt treated the medicine sodium salicylate with acetyl chloride to produce acetylsalicylic acid for the first time. For the next 50 years, other chemists established the chemical structure and devised more efficient production methods.:6975Aspirin is one of the most widely used medications globally, with an estimated 40,000 tonnes (44,000 tons) (50 to 120 billion pills) consumed each year. It is on the World Health Organization's List of Essential Medicines. It is available as a generic medication. In 2019, it was the 38th most commonly prescribed medication in the United States, with more than 18 million prescriptions.

Bayer AG (; German: [ˈbaɪɐ]) is a German multinational pharmaceutical and life sciences company and one of the largest pharmaceutical companies in the world. Headquartered in Leverkusen, Bayer's areas of business include pharmaceuticals; consumer healthcare products, agricultural chemicals, seeds and biotechnology products. The company is a component of the Euro Stoxx 50 stock market index.Bayer was founded in 1863 in Barmen as a partnership between dye salesman Friedrich Bayer and dyer Friedrich Weskott. As was common in this era, the company was established as a dyestuffs producer. The versatility of aniline chemistry led Bayer to expand their business into other areas, and in 1899 Bayer launched the compound acetylsalicylic acid under the trademarked name Aspirin. In 1904 Bayer received a trademark for the "Bayer Cross" logo, which was subsequently stamped onto each aspirin tablet, creating an iconic product that is still sold by Bayer. Other commonly known products initially commercialized by Bayer include heroin, phenobarbitol, polyurethanes, and polycarbonates.

In 1925 Bayer merged with five other German companies to form IG Farben, creating the world's largest chemical and pharmaceutical company. Following World War II, the Allied Control Council seized IG Farben's assets because of its role in the Nazi war effort and involvement in the Holocaust, which included using slave labour from concentration camps, the purchase of humans for dangerous medical testing, and production of Zyklon B, a chemical used in gas chambers. In 1951 IG Farben was split into its constituent companies, and Bayer was reincorporated as Farbenfabriken Bayer AG. Bayer played a key role in the Wirtschaftswunder in post-war West Germany, quickly regaining its position as one of the world's largest chemical and pharmaceutical corporations.