Blogger and internal medicine-primary care resident Ishani Ganguli posted this ad, created by Britain's National Health Service, meant to  dissuade patients from taking their relatively minor health problems to the emergency room.

The ad is part of a Choose Better campaign, which aims to encourage treatment of coughs and colds at a primary care physician's office or, in some cases, the patients' own home.

Here in the United States, emergency room trips have steadily grown, from 67 million visits in 1996 to 119 million trips in 2008, and ER trips can be expensive. One study back in 1994 found that treating non-emergency conditions in an emergency department setting contributed $5 billion to $7 billion in excess costs over the course of a year.

A campaign to reduce emergency room trips could be one way to drive down health spending, but the evidence we have on why people go to the emergency room suggests this type of approach may not succeed.

In 2001 and 2002, researchers at Mount Sinai School of Medicine interviewed hundreds of emergency department patients. They did so at 28 different hospitals, in order to achieve geographic diversity in their results. Just about everybody who turned up there—95.9 percent of the 1,547 patients interviewed—said they had a "medical emergency" that was too threatening to be treated elsewhere.

Separate research suggests that these patients are likely overestimating the severity of their medical needs. A recent analysis from the Center for Studying Health System Change found that about half of emergency room trips are "emergent" or "semi-emergent," needing to be treated within an hour.

For a patient though, it's hard to know what counts as emergent and what doesn't—that's the whole point of turning up at the doctor's office in the first place. The person with a cough or stomachache isn't quite sure how serious the condition is. In an article on emergency department overuse, Drs. Zachary Meisel and Jesse Pines write about the "information asymmetry" when patients turn up at the emergency department.

"The truth is, many people don't have a good way to judge whether a headache or fever is a true medical emergency," they write. "Heralded medical stories may contribute: Think about the coverage about Tim Russert's sudden death from a heart attack right after receiving a clean bill of health from his cardiologist. Now say you're the one feeling lightheaded: How do you know you're not going to just drop dead?"

Emergency room advertising in the United States, meanwhile, takes a bit of a different flavor. Private hospitals attempt to lure customers in by posting their wait times on billboards, advertising a short wait to receive care.