GlaxoSmithKline plc is a British multinational pharmaceutical company headquartered in Brentford, London. Established in 2000 by a merger of Glaxo Wellcome and SmithKline Beecham, GSK was the world's sixth largest pharmaceutical company as of 2015, after Pfizer, Novartis, Merck, Hoffmann-La Roche and Sanofi.