(CNN) -- Nathan Cox-Reed has a toothache.
He thinks he needs a root canal, but the full-time student, 22, is uninsured. He can't afford a trip to the dentist.
"I'm only working 30 hours a week. I wouldn't have enough money to do something like that," said Cox-Reed, a film and video student at Columbia College in Chicago.
While many young adults are now covered by the Affordable Care Act, able to remain on their parents' insurance until age 26, the rules are different for those like Cox-Reed, who grew up in the foster care system.
There are more than 400,000 children in foster care in the United States, the Department of Health and Human Services said last year. All are provided with health care coverage as long as they are wards of the state.
When foster kids turn 18, they age out of the system and instantly lose their coverage.
That's about to change, when another part of Obamacare takes effect on January 1, 2014. Medicaid coverage will be extended for former foster youth until they reach 26, as long as the individual was in foster care and enrolled in Medicaid until the age of 18.
"I definitely think it would be a big relief, and I would definitely feel more secure as far as my health goes," Cox-Reed said.
But there's a catch. Cox-Reed has dreams of traveling across the nation and becoming a filmmaker. A future relocation could jeopardize his medical coverage.
States will only be required to keep former foster children on Medicaid if they continue to reside in the state where they were in foster care originally.
This part of the provision is "an incredibly troubling aspect," said Washington attorney Brooke Lehmann, who founded the child and family advocacy group Childworks. Young adults can be highly mobile as they move for educational purposes, job opportunities and a host of other reasons, she said.
"You can't be on a film set if you are uninsured," Cox-Reed said. "You could get hurt. I definitely think [being uninsured] is limiting and it's a letdown, because what if I do get a job out of state? I might not be able to take it."
While "it's a great provision," said Joan Alker, co-executive director of the Georgetown Center for Children and Families and a professor at the Georgetown University Health Policy Institute, limiting extended Medicaid enrollment because of relocation could threaten the provision's effectiveness.
She, Lehmann and other child advocates wrote a letter, circulated by the Children's Defense Fund, to the Centers for Medicare & Medicaid Services at the Department of Health and Human Services, which is working on the revisions before this part of the ACA is implemented in 2014, asking them to reconsider this limitation.
Even if this part of the ACA is reinterpreted, former foster children like Cox-Reed may face other challenges with the existing system -- namely, not all doctors and dentists accept Medicaid.
Vicki Rodriguez, 20, who grew up in foster care in Illinois, found that out the hard way. She needs someone to remove her wisdom teeth, but can't find a single dentist who will take her Medicaid card and she can't afford to pay for it on her own.
"My tooth on my left side is coming in sideways, and it's pushing my other teeth forward," she said. "I can't go anywhere because it could cost about $3,000 [including] the X-rays and everything else that they have to do before."
Rodriguez believes having coverage extended until she reaches 26 would be "really awesome," but noted that even those with coverage are limited.
"People think, 'Oh yeah, the foster kids have everything covered for them, they can get everything done with their medical card,' but it's not true," she said. "There's really a lot that we can't do."
Lehmann recalls one of her past clients, who she said was "actively psychotic and suicidal": It took two years for her to obtain medication, Lehmann said, because of the difficulty in finding a psychiatrist who would take Medicaid.
"The reimbursement rates are abysmal" for providers, she said.
Health and Human Services has not responded to the child advocates' letter and did not respond to repeated requests for comment from CNN on the matter.
The advocates, including Alker and Lehmann, say enrollment in Medicaid should be faster and easier to ensure all eligible young adults are enrolled without obstacles.
An outreach campaign could inform eligible individuals so former foster youth are aware of the opportunity, they say.
"States are still trying to figure out how these former foster youth can be identified, how they can let young adults know that this benefit is open to them and how they can enroll," Lehmann said.
CNN's Jen Christensen contributed to this report.