Antibiotics don't help a chest cold much — even if you're coughing up icky green gunk, a new study shows.

The new findings don't apply to people with underlying lung disease. These patients probably do benefit from antibiotics. But the rest of us do not, find Paul Little, MD, professor of medicine at England's University of Southampton, and colleagues.

Little's team studied some 800 otherwise healthy people, aged 3 years and older, seeing a doctor for a lower respiratory infection. Doctors often call this bronchitis. Most of us know it as a chest cold.

The researchers gave some of the patients antibiotics right away. Others didn't get antibiotics at all. A third group got a prescription for antibiotics, but it was left in a box at the reception desk. They could get the prescription at any time but were advised to wait 14 days.

The bottom line: Nobody got better much faster than anybody else did. On average, patients already had a cough for nine days before they saw a doctor. It took about 12 more days for patients' coughs to get completely better — although one in four patients had a cough lasting 17 more days or longer. The findings appear in the June 22/29 issue of The Journal of the American Medical Association.

"Doctors have unfortunately been doling out antibiotics believing they will help — but antibiotics do not seem to be the answer," Little tells WebMD. "Antibiotics may make a difference of a day in an illness lasting three weeks. I tell patients, 'In your case, antibiotics will probably not make a difference — and you have to suffer their side effects if you take them.'"

Read WebMD's "Is Cough Medicine Effective?"

Chest Colds Linger Longer

Modern medical science has made huge strides in understanding and treating a wide variety of diseases. Yet surprisingly little is known about the common illnesses that plague us.

That's why Little's study is so important, says Mark H. Ebell, MD, deputy editor of American Family Physician and associate professor at Michigan State University. He says the findings aren't just a surprise to patients — they're a surprise to doctors, too.

For example, Ebell notes, doctors generally thought the cough from a chest cold lasted about a week. Surprisingly, Little's team finds that these coughs last for about three weeks — and often last a month. And it's also a surprise to many doctors that antibiotics really don't help otherwise healthy patients with chest colds.

"I hope this will educate doctors about the limits of antibiotics for treating cough," Ebell tells WebMD. "It is very hard for doctors to learn the limits of their own informal observations in practice and to lean that sometimes the studies are right and they are wrong."

Read WebMD's "Fewer Unnecessary Antibiotics Prescribed"

What About Children?

It's one thing to accept the news that antibiotics won't help our own chest colds. But won't they at least help our coughing kids?

No, Little and colleagues find. The otherwise healthy children enrolled in their study did the same as adults.

That confirms what some pediatricians already suspected, says Michael J. Light, MD, chairman of the American Academy of Pediatrics' pulmonology section and professor of clinical pediatrics at the University of Miami in Florida.

"The majority of kids don't need antibiotics unless they have a definite pneumonia," Light tells WebMD. "Lower respiratory infections in kids are common and they go on for a long time. We pediatricians take notice of coughs that go on beyond five or seven days, and those are the ones we tend to treat with antibiotics. But this study clearly shows that probably isn't the right approach. Waiting it out is correct."

Despite this reassurance, Light says parents should be aware of signs that children need help right away.

"If a cough lasts for more than several days, it is not an unreasonable thing to see the doctor," Light says. "If children have a fever that persists, if they seem to be breathing significantly faster than usual, this needs to be checked out."

A child that has difficulty breathing and begins to turn blue should immediately be taken to an emergency room.

Read WebMD's "Call in Sick or Go to Work? Here's Some Advice"

Danger Signs

Sometimes you really do need to see a doctor for a bad cough. It could be pneumonia. Signs of pneumonia are:

— Shortness of breath

— High fever

— Rapid breathing

— Coughing rusty-colored or bloody sputum

— Feeling very weak or tired

If you have any of these symptoms, it's a good idea to see a doctor.

"Basically, any time someone is worried we want them to come in," Ebell says. "There are other reasons to worry about a cough besides chest colds. Coughs can be caused by acid reflux, allergies, sinus infections, asthma, and other things we can treat. Any cough lasting more than two weeks should be evaluated by a doctor unless it is definitely getting better and almost gone."

By Daniel J. DeNoon, reviewed by Brunilda Nazario, MD

SOURCES: Little, P. The Journal of the American Medical Association, June 22/29, 2005; vol 293: pp 3029-3035. Paul Little, MD, professor of medicine, University of Southampton, England. Michael J. Light, MD, chairman, American Academy of Pediatrics Pulmonology Section; professor of clinical pediatrics, University of Miami, Florida. Mark H. Ebell, MD, deputy editor, American Family Physician; associate professor, Michigan State University, East Lansing, Mich.; and physician, Athens Primary Care, Athens, Ga.