It has several blockbuster products, including cancer immunotherapy, anti-diabetic medications, and vaccines for HPV and chickenpox, each generating significant revenue as of 2020.

[29] Merck perfected the chemical process of deriving morphine from opium and later introduced cocaine, used to treat sinus problems and to add to beverages to boost energy levels.

[41][42] In 1929, H. K. Mulford Company merged with Sharp and Dohme, Inc. and brought vaccine technology, including immunization of cavalry horses in World War I and delivery of a diphtheria antitoxin to Merck & Co.

At the time of its discovery, sanatoriums for the isolation of tuberculosis-infected people were a ubiquitous feature of cities in developed countries, with 50% dying within 5 years of admission.

[45] In the 1950s, thiazide diuretics were developed by Merck scientists Karl H. Beyer, James M. Sprague, John E. Baer, and Frederick C. Novello[46] and led to the marketing of the first drug of this class, chlorothiazide, under the trade name Duiril in 1958.

[40] In 1965, Merck & Co. acquired Charles Frosst Ltd. of Montreal (founded 1899), creating Merck-Frosst Canada, Inc., as its Canadian subsidiary and pharmaceutical research facility.

[63] Merck scientist William C. Campbell and Satoshi Ōmura developed ivermectin for veterinary use in 1981, and later put it to human use against Onchocerciasis in 1987–88 with the name Mectizan;[64] today the compound is used against river blindness, lymphatic filariasis, scabies and other parasitic infections.

[70] In 1991, Merck's Kelco subsidiary was responsible for volatile organic compound (VOC) emission pollution in the San Diego area.

[99] In July 2016, the company acquired Afferent Pharmaceuticals, developer of a candidate used to block P2RX3 receptors, for approximately $1 billion, plus up to $750 million in milestone payments.

[103] In September 2017, the company announced it would acquire Rigontec, developer of a candidate to target the retinoic acid-inducible gene I pathway, for $554 million.

[104][105] In October 2017, the company granted the inaugural Merck-AGITG Clinical Research Fellowship in Gastro-Intestinal (GI) Cancer to David Lau, a professional in Melbourne, Australia.

[108] In 2018, Merck began the submission process for a Biologics License Application to the Food and Drug Administration under the Breakthrough Therapy Designation for an investigational vaccine, called V920, to fight the Zaire strain of the Ebola virus.

[112] In May 2019, Merck announced it would acquire Peloton Therapeutics, developer of a HIF-2alpha inhibitor for Von Hippel–Lindau disease-associated renal cell carcinoma, for up to $2.2 billion.

[118] In June 2020, Merck acquired Themis Bioscience, a company focused on vaccines and immune-modulation therapies for infectious diseases including COVID-19 and cancer.

[119][120][121] Also in June 2020, Merck Animal Health acquired Quantified Ag, a data and analytics company that monitors cattle body temperature and movement in order to detect illness early.

[124][125] In November 2020, Merck announced it would acquire VelosBio for $2.75 billion, developer of VLS-101, an antibody-drug conjugate designed to target Tyrosine kinase-like orphan receptor 1 (ROR1) in both hematological and solid tumors.

[129] In March 2021, Merck Head of Corporate Affairs Petra Wicklandt represented the company at the Munich Security Conference, where she participated in a tabletop exercise simulating the public health response to the release of a weaponized strain of monkeypox.

[131][132][133] In June 2021, the U.S. government agreed to spend $1.2 billion to purchase 1.7 million doses of Molnupiravir, a Merck product, if it were to be approved by regulators to treat COVID-19.

[144] In December 2023, Merck announced it had partnered with Owkin to develop artificial intelligence-powered digital pathology diagnostics that could be used to identify patients suitable for immunotherapies.

The aim is to come up with tools that can pre-screen patients with four tumour types for the MSI-H biomarker, namely endometrial, gastric, small intestinal, and biliary cancers.

Up to that point, the World Health Organization had fought the disease through the use of insecticides to lower the population of its primary vector, the Black Fly.

Such compounds were expected to cause less gastrointestinal bleeding than older anti-inflammatory drugs such as naproxen, which were associated with 20,000 hospitalizations and 2000 deaths each year.

[168] On September 23, 2004, Merck received information about results from a clinical trial it was conducting that included findings of increased risk of heart attacks among Vioxx users who had been using the medication for over eighteen months.

An analysis for the period 1999–2004, based on U.S. Medical Expenditure Survey data, reported that Vioxx was associated with 46,783 heart attacks, and along with the other popular COX-2 inhibitor Celebrex, an estimated 26,603 deaths from both.

[170][171] About 50,000 people sued Merck, claiming they or their family members had suffered medical problems such as heart attacks or strokes after taking Vioxx.

[183][184] In December 2013, Merck agreed to pay a total of $27.7 million to 1,200 plaintiffs in a class action lawsuit alleging that the company's osteoporosis drug had caused them to develop osteonecrosis of the jaw.

[186] A fraud investigation by the United States Department of Justice began in 2000 when allegations were brought in two separate lawsuits filed by whistleblowers under the False Claims Act.

[198] Merck & Co. once used methylene chloride, an animal carcinogen on the United States Environmental Protection Agency's list of pollutants, as a solvent in some of its manufacturing processes.

Merck has also modified its equipment to protect the environment, installing a distributed control system that coordinates chemical reactions more efficiently and expedites manufacturing by 50 percent, eliminating the need for the disposal and storage of harmful waste.

[200] Merck is a contributing member of AcademyHealth, a health research advocacy network that also includes government agencies, universities, pharmaceutical companies and lobbying groups.