NEW YORK (Reuters) - Boston Red Sox slugger David Ortiz said taking nutritional supplements may have caused him to land on a 2003 list of Major League players who tested positive for performance-enhancing substances.

The five-times All-Star said he was "a little bit careless back in those days when I was buying supplements and vitamins over the counter."

"But I never ... use(d) steroids," he told a news conference at Yankee Stadium on Saturday before Boston's game against the Yankees.

Ortiz and then-team mate Manny Ramirez, who helped the Boston Red Sox end an 86-year World Series title drought in 2004, tested positive for performance-enhancing drugs in 2003, The New York Times reported on July 30.

The burly, 33-year-old Ortiz was among the 104 players who tested positive in what was meant to be confidential testing, the Times said, citing unidentified lawyers with knowledge of the results.

"I never thought that buying supplements and vitamins, it was going to hurt anybody's feelings and that happened," said Ortiz, a native of the Dominican Republic.

"And I'm sorry about that. But that wasn't my case, my situation."

The Red Sox backed Ortiz, known as "Big Papi," and said there were "substantial uncertainties and ambiguity surrounding the list" of players who allegedly tested positive in 2003.

"David Ortiz is a team leader and his contributions on the field and in the community have earned him respect and a special place in the hearts of Red Sox Nation," the team said in a statement.

Baseball's 2003 testing was conducted in agreement with the players' union to determine if there would be mandatory random testing in 2004, which was instituted after a threshold of 5 percent positive tests was exceeded.

Results of the confidential testing were seized by the U.S. government in conjunction with an investigation into the distribution of performance-enhancing drugs to professional athletes.

(Writing by Steve Ginsburg in Washington; Editing by Sonia Oxley)