The United States has been a paramount influence on the history of Glaxo Laboratories. From the original diversification of 1923, into vitamins via the Zucker patents, which turned the then small Glaxo department towards pharmaceutical work, through to the antibiotic deep fermentation techniques in the 1940s and 1950s, the major technological and marketing stimuli to Glaxo's growth have come from the United States.

The company's relations with US pharmaceutical manufacturers were vital to its penicillin and streptomycin production during the war and in the period immediately afterwards. Glaxo's growth and particularly its strategic acquisitions in the late 1950s and early 1960s were a response to US competition. The technological superiority of the US pharmaceutical corporations not only ensured the continuation of licensing arrangements in the later 1950s and early 1960s, but also effectively inhibited Glaxo from attempting to establish a presence in the US itself. American success in overseas markets, particularly those most adjacent to itself, also limited Glaxo's operations. In both Canada, where British influence had for a long time predominated, and in the countries of South America, Glaxo's post-war operations were curtailed both by US competition and by the arrangements made by the parent company with some of the major US pharmaceutical houses. In some of the South American countries, particularly Argentina, Glaxo was well established before the war. In Canada, by contrast, Glaxo started almost from scratch in 1950 and paid the penalty of coming late into a market already dominated by the large US pharmaceutical corporations.