Pfizer is an American multinational pharmaceutical corporation headquartered in New York City, and with its research headquarters in Groton, Connecticut, United States. It is one of the world's largest pharmaceutical companies by revenues.

Pfizer develops and produces medicines and vaccines for a wide range of medical disciplines, including immunology, oncology, cardiology, diabetology/endocrinology, and neurology. Pfizer's products include the blockbuster drug Lipitor (atorvastatin), used to lower LDL blood cholesterol; Lyrica (pregabalin for neuropathic pain/fibromyalgia); Diflucan (fluconazole), an oral antifungal medication; Zithromax (azithromycin), an antibiotic; Viagra (sildenafil, for erectile dysfunction); and Celebrex/Celebra (celecoxib), an anti-inflammatory drug.

Pfizer was founded by cousins Charles Pfizer and Charles Erhart in New York City in 1849 as a manufacturer of fine chemicals. Pfizer's discovery of Terramycin (oxytetracycline) in 1950 put it on a path towards becoming a research-based pharmaceutical company. Pfizer has made numerous acquisitions, including Warner–Lambert in 2000, Pharmacia in 2003 and Wyeth in 2009. The Wyeth acquisition was the largest of the three at $68 billion. Pfizer is listed on the New York Stock Exchange, and its shares have been a component of the Dow Jones Industrial Average since April 2004.

In August 2014, it was suggested that Pfizer may attempt to acquire GlaxoSmithKline after failing to buy AstraZeneca.