Even so, he said, "we will implement" the law, although he conceded there will be glitches along the way.
"Despite all the hue and cry and sky-is-falling predictions about this stuff, if you've already got health insurance, then the part of Obamacare that affects you is already in place," he added.
The first question to Obama concerned Syria and the reported use of chemical weapons.
Administration officials said recently that intelligence analysts had "varying degrees of confidence" in a conclusion that Assad's government has deployed sarin gas against civilians.
Obama said the administration was using all its resources to determine the facts about a weapon that he has said would be a "game changer" for U.S. policy in the war.
"If we end up rushing to judgment without hard, effective evidence ... we can find ourselves in a position where we can't marshal the international community in support what we do," he said. "It's important for us to do this in a prudent way."
The administration long ago called for Assad to step down and pave the way for a new government, but Obama has resisted calls from some Republicans in Congress to send U.S. military aid to the rebels and perhaps commit U.S. military resources directly.
The hour-long news conference concluded with a post-script.
Obama had stepped away from the lectern when he heard a shouted question about Jason Collins, the professional basketball player who made a pioneering announcement on Monday that he is gay.
Obama said he had spoken with Collins and "told him I couldn't be prouder of him."
"One of the extraordinary measures of progress that we've seen in this country has been the recognition that the LGBT community deserves full equality, not just partial equality, not just tolerance but a recognition that they're wholly a part of the American family," he said.
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