Two ingredients commonly used in cough syrup are no better than sugar water in suppressing night-time coughing in children, according to a study published on Tuesday.

The two ingredients are dextromethorphan - often listed on labels as “DM” - and diphenhydramine, an antihistamine. The former is the most common nonprescription cough suppressant on the U.S. market, and commonly abused by adolescents who try to get high on cough medicine.

“Consumers spend billions of dollars each year on over-the-counter medications for cough,” said Ian Paul, a physician and assistant professor of pediatrics at Penn State Children’s Hospital.

“Our study showed that the two ingredients used in most over-the-counter medications were no better than a placebo ... in providing night time relief for children with cough and sleep difficulty as a result of upper respiratory infection,” he added.

Paul was the chief author of the study appearing in the July issue of Pediatrics, the journal of the American Academy of Pediatrics.

The findings were based on 100 children aged 2 to 18 with upper respiratory infections. Their parents were quizzed about the severity of the children’s cough and how well both parents and children slept the previous night.

In the evening of the day the parents were questioned the children were given either one of the commercial preparations or an inert placebo - in this case simple syrup.

“There was a significant improvement for all symptoms over the previous night, which should reassure clinicians and parents that, regardless of treatment, the natural history of an upper respiratory infection favors resolution of symptoms with time,” Paul said.

In an interview, Paul said the improvement in symptoms across-the-board was due to both the natural progression of an infection easing one day to the next and the well-documented “placebo effect,” where symptoms diminish because a patient believes a treatment is helping.

He said the sleep of both parents and children improved but the improvement was the same in the group given sugar syrup as for the children given the drugs.

Asked what parents should do, Paul said “my advice has been to do things that are harmless but could help - saline nose drops, good hydration and humidified air.”