Major customers

In the United States, we sell primarily to pharmaceutical wholesale distributors that we utilize as the principal means of distributing our products to healthcare providers. Outside the United States, we sell principally to healthcare providers and/or pharmaceutical wholesale distributors depending on the distribution practice in each country. We monitor the financial condition of our larger customers and limit our credit exposure by setting credit limits and, in certain circumstances, by requiring letters of credit or obtaining credit insurance.

We had product sales to three customers, each of them accounting for more than 10% of total revenues for each of the years ended December 31, 2018, 2017 and 2016. For the year ended December 31, 2018, on a combined basis, these customers accounted for 84% and 98% of total gross revenues and U.S. gross product sales, respectively, as shown in the following table. Certain information with respect to these customers was as follows (dollar amounts in millions):



	Years ended December 31,
	2018			2017			2016
AmerisourceBergen Corporation:
Gross product sales	$	12,091		$	10,742		$	10,100
% of total gross revenues	33		%	31		%	31		%
% of U.S. gross product sales	39		%	37		%	38		%
McKesson Corporation:
Gross product sales	$	11,434		$	10,625		$	9,710
% of total gross revenues	31		%	30		%	30		%
% of U.S. gross product sales	35		%	35		%	34		%
Cardinal Health, Inc.:
Gross product sales	$	7,475		$	7,049		$	6,520
% of total gross revenues	20		%	20		%	20		%
% of U.S. gross product sales	24		%	24		%	24		%

As of December 31, 2018 and 2017, amounts due from these three customers each exceeded 10% of gross trade receivables and accounted for 76% and 75%, respectively, of net trade receivables on a combined basis. As of December 31, 2018 and 2017, 23% and 25%, respectively, of trade receivables, net, were due from customers located outside the United States—primarily in Europe. Our total allowance for doubtful accounts as of December 31, 2018 and 2017 was not material.