Starting in January, Americans eligible for Medicare coverage for their medical expenses will also be eligible for prescription drug coverage in the new Medicare Part D program.

But some seniors are struggling with the registration process, and they have no shortage of questions about what's covered and what's not.

Click in the video box to the right to watch a report by FOX News' Major Garrett.

Private insurers are rushing to answer those questions, as they did at a recent signup seminar in Aurora, Ill. The upshot — sometimes lower prices come with a catch.

In January, the new drug coverage plan will be open to 42 million beneficiaries. Dozens of drug plans are available in each state and variations abound — different prices for different drugs, generics in some locations, brand name choices in others, sometimes a mixture of both.

"Our seniors, with so many plans to choose from, are confused. In some cases, they're bewildered," Sen. Bill Nelson, D-Fla., said last week.

Republican architects of the drug coverage plan disagree

"I for one am tired of people on the other side seeming to have lack of confidence in our American senior citizens who are often well-informed about the choices they can make and do make good choices," said Sen. Charles Grassley, R-Iowa.

Medicare's Web site is one resource to navigate the choices. The senior group AARP and health research groups like the Kaiser Family Foundation also offer other resources. Sign-up ends May 15, 2006, with the promise that the end result will be lower costs for everyone.

"For a typical beneficiary who is going to spend over $2,000 on drugs in 2006, you can save more than 50 percent on medications," said Medicare administrator Mark McClellan.

Once seniors sign up, they can't change providers for one year. Medicare could penalize seniors without drug coverage who don't sign up by May.

"If you wait for health insurance until you really need it, it's probably going to cost you more," McClellan said.

Critics say the plan will solve one problem — covering poor seniors — while it will create another one — gradually killing off more generous private-sector retiree drug plans.

"Why would any company that is interested in the bottom line continue to pay for something if the government is going to pay for it?" asked Derek Hunter, a policy analyst for the Heritage Foundation.

Advocates say there's no proof private companies will drop drug coverage. They say the plan has already proven these same critics wrong in one area, by delivering lower costs than forecast.

"Premiums have come in on average 15 percent lower than expected, the cost to the government per person is going to be at least 15 percent lower than expected," McClellan said.

McClellan told FOX News that he now expects the drug benefit will cost less than the projected 10-year price tag of $858 billion. He also predicted the drug benefit will lower the costs Medicare pays for doctor visits, hospital visits and rehabilitation because drug therapy will now treat underlying health problems before they become catastrophic both to the patient and Medicare's bottom line.