Two active ingredients in many cough medicines do little to speed recovery, US researchers have suggested.
The dummy syrup was equally effective
Night-time cough and sleep quality were no better with cough mixtures than with a simple, non-medicated syrup.
The team from Penn State Children's Hospital, writing in Pediatrics, say their findings call in to question if these medicines should be used.
UK experts said they would do no harm to those who took them - but said they also did not provide any benefit.
Previous research has questioned the effectiveness of common cough medicines that can be bought over-the-counter at pharmacies.
Dr Paul and colleagues set out to test two of the main ingredients found in cough medicines head-to-head with a dummy syrup containing no medication.
They split 100 children with cough caused by upper respiratory tract infections into three groups.
One group was given a syrup containing the active ingredient dextromethorphan, the second was given a syrup containing the active ingredient diphenhydramine and the third was given the dummy syrup.
The children had all been suffering from cough for about four days before the study.
The children's parents were asked to give the syrup to their child 30 minutes before bedtime.
The were also asked to rate their children's symptoms the night before the treatment and the night immediately after the treatment had been given.
Neither of the two active ingredients appeared to be better than the simple syrup.
All three groups showed similar improvements in bothersome nature of cough, severity of cough and impact on the child's sleep.
The children who received the dummy syrup had less frequent cough afterwards than those who received one of the active ingredients.
Dr Paul said the improvements in symptoms regardless of the treatment should reassure doctors and parents that children with these types of night-time cough get better with time.
"This study, however, questions whether over-the-counter medications have a place in the treatment of these illnesses for children," he said.
Neither medications improved the parent's sleep, he noted.
"Parents often look particularly hard for ways to calm their child's cough at night because parents, too, need sleep to get through their daytime activities.
"Our study specifically evaluated this variable and showed that, not only did children's sleep not improve, but parents' sleep didn't improve when their child received active medication versus placebo," he said.
Dr Paul suggested doctors should be cautious about recommending cough medicines and consider the cost and potential for adverse effects compared with any benefits.
The Proprietary Association of Great Britain said the results might not reflect the real-life situation because the researchers looked at only 100 children over one night.
"People have been treating themselves and their children satisfactorily with cough medicines bought over-the-counter for many decades," said a spokeswoman.
"The PAGB agrees with the study findings that if people only have a minor cough they should start off with treatments that merely soothe the throat.
"If however the cough persists and gets more frequent, it is advisable to visit the pharmacist for advice on stronger medicines to treat the cough," she said.
Professor Andrew Peacock of the British Thoracic Society said: "Over-the-counter cough medicines don't do any harm to people who take them, but as this latest research shows, neither do they have any medical benefits beyond those of non-medicated syrup."
Dr Harvey Marcovitch, a spokesman for the Royal College of Paediatrics and Child Health, said: "For children with their usual tickly coughs that they get all the time there really isn't much evidence that they do any more good than any sort of warm drink."