Millions of low-income older Americans may be temporarily unable to get their prescription medications as they enroll in the new Medicare drug benefit, according to several advocacy groups.

Beginning Jan. 1, 6.4 million people whose drug costs are now covered by Medicaid, the government program that pays medical bills for low-income, elderly and disabled people with no other access to care, instead will get their drugs through Medicare, the federal health insurance program for the elderly.

The Medicare agency sent such "dual-eligibles" letters in October informing them of which Medicare drug plan they would automatically be enrolled in if they did not pick a plan by Dec. 31.

Advocates are worried that without a smooth transition some recipients could lack access to crucial drugs for days or weeks. Eight nonprofit groups filed a lawsuit against the government Monday asking a federal judge to guarantee that does not happen.

"We think there is a grave danger," said Robert M. Hayes, president of the Medicare Rights Center, one of the plaintiffs. "The data exchanges will have inevitable breakdowns. And given the poverty, age and disability of these folks, they will largely be unable to immediately navigate a whole new system."

In a report this week, the Government Accountability Office also noted that the switch "could create difficulties ensuring that prescriptions for this vulnerable population are filled."

Medicare officials say they have taken sufficient precautions to ensure there are no gaps, pointing to the automatic enrollment of recipients in new Medicare drug plans as an example.

-- Christopher Lee

Low-income elderly people could see a temporary gap in their prescription drug coverage after Jan. 1.