Senators have delayed the confirmation hearing for Ronny Jackson, the White House physician President Donald Trump picked to lead the Department of Veterans Affairs, as they investigate allegations against the doctor.

Jackson faces claims of overprescribing medications and drinking on the job, The New York Times reported Tuesday. It is unclear when specifically the claims stem from, but the doctor has served under not only Trump but also Presidents Barack Obama and George W. Bush.

The Senate Veterans Affairs Committee had planned to hold a hearing Wednesday, but the panel's top lawmakers confirmed Tuesday that it would be delayed. The committee's ranking member, Sen. Jon Tester, D-Mont., told reporters Tuesday that the timing of a hearing would depend on what senators find out.

Tester would not confirm what allegations against Jackson surfaced. He said "there are a lot of things we're looking into."

Separately, Tester and committee Chairman Johnny Isakson, R-Ga., sent a letter to the White House requesting documents related to Jackson's service and any allegations against him.

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Committee member Sen. Mike Rounds, R-S.D., called the claims "unsubstantiated."

Jackson, a rear admiral in the Navy, did not explicitly deny the accusations when questioned by reporters on Capitol Hill on Tuesday. He said he is "looking forward to the hearing," where he plans to answer questions.

The White House also defended Jackson on Tuesday.

"Admiral Jackson's record of strong, decisive leadership is exactly what's needed at the V.A. to ensure our veterans receive the benefits they deserve," deputy press secretary Hogan Gidley said in a statement.

Jackson already faced serious questions about his qualifications to lead the VA, a sprawling and troubled agency.