AbbVie and Allergan said they still expect the deal to be finalized by early 2020, and partnered with the antitrust regulator.
The review of the deal, which combines two of the world's largest pharmaceutical companies, follows a request from a dozen advocacy groups and unions to block the purchase.
In a letter seen by Reuters, the groups noted that the deal would create the fourth largest drug company at a time when rising drug prices have become a hot political issue.
AbbVie said the FTC's second request for information was expected.
The deal to buy Allergan, announced in June, came when Abbvie faced pressure to diversify the portfolio, and as its success drug Humira, the world's best-selling drug, faces competition from cheaper versions.
(Reporting by Manas Mishra in Bengaluru; Editing by Shounak Dasgupta)