A new study suggests menthol cigarettes could be more harmful to smokers than regular cigarettes. Researchers found menthol cigarette smokers had more trips to the emergency room and more hospitalizations or treatment for severe lung disease flare-ups.

These flare-ups included difficulty breathing and a major increase in phlegm production, Reuters reports.

“We were surprised that menthol smokers, compared to non-menthol cigarette smokers, reported more severe exacerbations and had a greater odds of experiencing severe exacerbations,” said study co-author Dr. Marilyn Foreman of the Morehouse School of Medicine in Atlanta.

The study included 3,758 menthol smokers and 1,941 regular smokers who had smoked at least 10 packs of cigarettes annually. Those who smoked menthol cigarettes were 29 percent more likely than regular cigarette smokers to have severe flare-ups of chronic obstructive pulmonary disease.

Menthol smokers performed more poorly on a test of how far they could walk in six minutes, and were more short of breath compared with regular cigarette smokers, the researchers reported in the journal Respirology.

Foreman said menthol may have an anesthetic effect on the airways, which could delay the diagnosis of worsening lung disease.

Earlier this year, a group of former top health officials urged tobacco companies to stop marketing and selling menthol cigarettes. The group included all of the living former U.S. Secretaries of Health, Surgeons General, and Directors of the Centers for Disease Control and Prevention.

The group, called the Citizens’ Commission to Protect the Truth, also called on the Obama Administration to allow the Food and Drug Administration to ban menthol flavoring in cigarettes.

The group called menthol “the spoonful of sugar that makes the deadly medicine these companies are selling go down.” They said there is overwhelming evidence that menthol disguises the harsh taste of tobacco, and makes it more difficult to quit smoking.