The first sulfonamide and the first systemically active antibacterial drug, forerunner of antibiotics,[8] Prontosil, was developed by a research team led by Gerhard Domagk in 1932 or 1933 at the Bayer Laboratories.

[11][12] 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.

[13] However, owing to the massive financial and reputational blows caused by ongoing litigation concerning Monsanto's herbicide Roundup, the deal is considered one of the worst corporate mergers in history.

[20] A further expansion in Elberfeld was impossible, so the company moved to the village Wiesdorf at Rhein and settled in the area of the alizarin producer Leverkus and Sons.

[22] Bayer's first major product was acetylsalicylic acid—first described by French chemist Charles Frederic Gerhardt in 1853[23]—a modification of salicylic acid or salicin, a folk remedy found in the bark of the willow plant.

[26] The term aspirin continued to be used in the US, UK and France for all brands of the drug,[26] but it is still a registered trademark of Bayer in over 80 countries, including Canada, Mexico, Germany and Switzerland.

Arthur Eichengrün, a Bayer chemist, said he was the first to discover an aspirin formulation that did not have the unpleasant side effects of nausea and gastric pain.

[25][31] Heroin (diacetylmorphine), now illegal as an addictive drug, was introduced as a non-addictive substitute for morphine,[32] and trademarked and marketed by Bayer from 1898 to 1910 as a cough suppressant and over-the-counter treatment for other common ailments, including pneumonia and tuberculosis.

[36] In 1903, Bayer licensed the patent for the hypnotic drug diethylbarbituric acid from its inventors Emil Fischer and Joseph von Mering.

[45] In the 1930s, Gerhard Domagk, director of Bayer's Institute of Pathology and Bacteriology, working with chemists Fritz Mietzsch and Joseph Klarer, discovered prontosil, the first commercially available antibacterial drug.

Despite being a convicted nazi war criminal,[11] Ter Meer was elected as chairman of Bayer AG's supervisory board in 1956, a position he retained until 1964.

[61] Helge Wehmeier, then CEO of Bayer, offered a public apology in 1995 to Elie Wiesel for the company's actions during World War II (1939–1945) and the Holocaust.

A review of the matter by the Medicines and Healthcare products Regulatory Agency in 2014 assessed the studies performed to date and found the evidence for adverse effects to be inconclusive.

In 1997, Bayer and the other three makers of such blood products agreed to pay $660 million to settle cases on behalf of more than 6,000 hemophiliacs infected in United States.

The side effect was rhabdomyolysis, causing kidney failure, which occurred with a tenfold greater frequency in patients treated with Baycol in comparison to those prescribed alternate medications of the statin class.

[67] Trasylol (aprotinin), used to control bleeding during major surgery, was withdrawn from the market worldwide in 2007 when reports of increased mortality emerged; it was later re-introduced in Europe but not in the US.

[131] In September 2015, Bayer spun out its $12.3 billion materials science division into a separate, publicly traded company called Covestro in which it retained about a 70% interest.

[138] In order to receive regulatory approval, Bayer agreed to divest a significant amount of its current agricultural assets to BASF in a series of deals.

[149] However, owing to ongoing litigation concerning the Monsanto's herbicide Roundup and the massive financial and reputational blows it has caused Bayer, the deal is considered one of the worst corporate mergers in history.

[175] The Pharmaceuticals Division focuses on prescription products, especially for women's healthcare and cardiology, and also on specialty therapeutics in the areas of oncology, hematology and ophthalmology.

[188] The key trends of Bayer are (as at the financial year ending December 31):[189][190] * without Covestro from 2017 on In 1904, the company founded the sports club TuS 04 ("Turn- und Spielverein der Farbenfabriken vorm.

[203] On 13 May 2019, a United States Superior Court Judge ordered Bayer to pay more than $ 2.5 billion in damages to a couple in California, both of whom contracted non-Hodgkin's Lymphoma, later cut to $87 million on appeal.

[208] The general consensus among national regulatory agencies, and the European Commission is that labeled usage of the herbicide poses no carcinogenic or genotoxic risk to humans.

"[205] On 17 June 2022, California-based United States Court of Appeals for the Ninth Circuit ordered the Environmental Protection Agency to reexamine this 2020 finding that glyphosate did not pose a health risk for people exposed to it by any means.

[214] In 2019, a federal jury in San Francisco CA sided with Bayer in a $600 million (US) class action suit alleging that the company misinformed consumers by promoting its One A Day vitamins as supporting cardiac health, vigorous immune systems and boosting user energy.

The suit was first filed as a nationwide class action; in 2017, the US District Court in San Francisco said subclasses of purchasers of the vitamin in Florida, New York, and California could act together.

Bayer misrepresented the results of its own research and knowingly supplied hemophilia medication tainted with HIV to patients in Asia and Latin America, without the precaution of heat treating the product, recommended for eliminating the risk.

[224] In June 2020, Bayer agreed to pay $800 million to settle lawsuits in a variety of jurisdictions which claimed contamination of public waterways with PCBs by Monsanto before 1978.

[206] On 25 November 2020, however, U.S. District Judge Fernando M. Olguin rejected Bayer's settlement offer, which was now at $650 million, and allowed Monsanto-related lawsuits involving PCB to proceed.

The judge called Bayer’s interpretation of the purchase agreement “the only reasonable one,” and said letting Merck “dump” cases would give the Rahway, New Jersey–based company an incentive to prolong or stall lawsuits.