(CBS NEWS) Doctors are worried that those who overuse ERs — dubbed “superusers” — are causing a severe burden on U.S. emergency departments.

“When things are crowded, people are rushed, they can feel the stress of extra patients waiting to be seen, and they tend to make more mistakes,” Dr. Jeff Dubin an emergency physician at MedStar Washington Hospital Center, and author of a new study on superusers, told CBS News.

Superusers are a small but expensive group: One large study conducted from 2009-2012 at a Los Angeles hospital noted that 99 percent of ER visitors come in fewer than 10 times a year, with nearly 67 percent coming in just once. But, the remaining 1 percent of visitors accounted for thousands of visits in one year.

Note: Read our discussion guidelines before commenting.