Experts debunk benefits of common cough syrups

For instance, one manufacturer claimed that their preparation "works deep down to loosen phlegm, clear bronchial congestion, and make your cough more productive."

Experts say the product's key active ingredient is guaifenesin, but the experts said the studies which are used to support the claims of its effectiveness are low quality. Nor has the company provided any evidence of effectiveness.

The active ingredients of another mixture are sugar alcohol (glycerol) and liquid sugar (sucrose), the 'Daily Mail' reported.

It also includes other sugars such as black treacle. The experts said the mixture is roughly half sugar and its effectiveness is "unproven".

The experts also found out that one week of the adult maximum dose of the Benylin Tickly Coughs mixture is equivalent to eating the sugar contained in five Mars bars.

The team was similarly sceptical about the claims made for Covonia Herbal Mucus Cough Syrup.

UK consumer campaigners Which? said companies must be honest about what these products, which are often expensive, can do, particularly when it comes to important issues like health.

"We spend billions on over-the-counter pharmacy products each year but we've found evidence of popular products making claims that our experts judged just aren't backed by sufficient evidence," a spokesman said.

Companies should be upfront with the evidence behind the claims they make so that consumers can make an informed decision," the spokesman said.

A number of studies have identified glucosamine, omega 3 and chondroitin as having beneficial effects in terms of protecting ageing joints. And various medicines have latched on to these claims in order to increase sales.

However, the European Food Safety Authority (EFSA) recently ruled the evidence was not strong enough to substantiate the claims.

The manufacturers of the products examined by Which? said that they all met the standards of the independent Medicines and Health-care products Regulatory Agency (MHRA), which requires evidence to demonstrate they work.