Sarah Sanders defends Jackson's 'impeccable' record

"Dr. Jackson's record as a White House physician has been impeccable," Sanders said. "Because he has worked within arm's reach of three presidents, he has in fact received more vetting than most nominees."

Sanders said Jackson has passed four background investigations, including a recent FBI investigation that is part of the vetting process for Cabinet nominees. She said the investigations "revealed no areas of concern."

Sanders did not say whether the most recent FBI background investigation took place before or after Trump tapped Jackson as the VA nominee.

Still, she insisted Jackson "a very thorough investigation and vetting process has taken place."

Four sources told CNN that during an overseas trip in 2015, Jackson, the White House physician, was intoxicated and banged on the hotel room door of a female employee. One source familiar with the allegation said the incident became so noisy that the Secret Service stopped him out of concern that he would wake then-President Barack Obama.

Sen. Jon Tester of Montana, the top Democrat on the Senate Veterans Affairs Committee, said the committee has spoken to some 20 retired and active members of the military about Jackson.

On Tuesday, Tester told CNN's Anderson Cooper that others who have come forward alleged that Jackson would hand out prescriptions "like candy" on overseas trips, distributing Ambien and Provigil down the aisle of the airplane.

Members of the Senate Veterans' Affairs Committee have been working through all of the allegations, but have not substantiated the claims, with little documentation available to corroborate them.