People may be wasting their money by buying over the counter cough mixtures according to researchers who said that the NHS might also have to reconsider advice to patients about taking them.
They said results of 15 trials involving 2,000 adult patients which compared the effects of the medicines on people with acute coughs with the effects of substances known to have no therapeutic effect.
In nine trials, the active treatment was no better than that of the placebos and in the remaining six, positive results were "of questionable clinical relevance", said Kurt Schoeder and Tom Fahey, of Bristol University's department of primary health care, in today's British Medical Journal.
The Proprietary Association of Great Britain, representing over the counter medicine manufacturers, encourages GPs and other health professionals to recommend their products as a first line treatment for coughs. Retail sales are thought to account for £94m a year.
The authors examined evidence for medicines using different mechanisms to tackle coughs, including expectorants, antihistamines and those using ingredients such as codeine.
They concluded that: "It remains unclear whether over the counter cough preparations are helpful in acute cough. We therefore cannot yet recommend these medicines as a first line treatment for cough associated with upper respiratory tract infection.
"The advice given by NHS Direct to use over the counter cough medicines should therefore be restricted until more evidence becomes available on their effectiveness."
They said their conclusions should be treated with caution and more research was needed including into whether the patients were satisfied with the medicines, whether they felt disturbance during the night, had side effects and how quickly they returned to normal daily life.
A PAGB spokesman said: "People have been treating themselves and their children satisfactorily with cough medicines bought over the counter for decades.
"Research carried out on behalf of the PAGB among consumers in 1997 found that in 90% of cases, cough medicines were described as effective and 90% of people would use the same product again to treat the same problem."
Regulators worldwide accepted the effectiveness of products. The PAGB spokesman said: "If cough symptoms persist, consumers should consult their doctor or pharmacist for advice. Parents should always check the label to see whether or not the product is suitable for use in children."
The Department of Health disputed the authors' claims over NHS Direct advice, saying it advised those complaining of a cough to consult a pharmacist.
