No prizes for guessing which condition plunges millions of us into misery each winter and triggers a desire for nothing more than bed, a hot water bottle and a large glass of warm whisky.

The common cold afflicts most adults two or three times a year and has triggered a pounds 300m industry in potions ranging from hot lemon drinks to decongestants. Yet cures for the cold remain as elusive as ever; the Common Cold Research Unit in Salisbury, which was closed in 1990, spent 30 years vainly trying to find one.

However, makers of a new product developed in the US are claiming that it can cut the duration of a cold by up to 60 per cent. The "Cold Killer", launched in a flurry of publicity this month, is a lozenge containing an "enteric zinc and silver complex" plus herbs such as elderberry extract, eucalyptus, horehound and wild cherry bark. According to the PR blurb, if the lozenges are sucked regularly "as soon as you feel a cold coming on", the zinc is delivered to the mucous membranes to kill the rhinoviruses, one of the two major groups which cause colds. The other ingredients reduce discomfort.

Can Cold Killers triumph where all previous efforts have failed? Zinc has long been known to have anti-viral properties in the lab, but research into its effect on colds in real people has always proved difficult to replicate. One drawback is that high concentrations of zinc taste disgusting - some trials collapsed when volunteers refused to continue taking it. Making the zinc more palatable by adding another ingredient appears to stop it from working. Once the zinc ions were chelated, or bound to molecules from another substance, they lost their charge.

A small breakthrough occurred earlier this year. A clinical trial published in July in the Annals of Internal Medicine, involving 100 volunteers, showed that those taking the compound zinc gluconate suffered cold symptoms for an average of four days, compared with 7.6 days in a placebo group. Zinc gluconate, concluded researchers from Hofstra University, New York State, can significantly reduce the duration of a cold, with benefits outweighing possible adverse effects such as mouth irritation and nausea.

Nobody quite knows how zinc fights a cold. One theory is that the zinc ions bind to the proteins in the virus to prevent it reproducing. The real puzzle is how enough zinc gets into the mucous membranes of the nose, where the virus resides. One study showed that sucking a zinc lozenge leads to most of the mineral being swallowed.

Most scientists concur that until further trials take place, it is too early to claim that zinc can "kill" cold viruses. One or two zinc compounds, sold as food supplements, are available as lozenges in Britain, but even a slight variation in the trial formulation means there might not be the same effect.

What else can you do to combat a cold? Paracetamol or aspirin are the first line of treatment for headache, fever and sore throat, while nasal decongestants and steam infusions are also useful. Antibiotics do not work on viruses (although they do fight off accompanying bacterial infections) and there is no clinical evidence that vitamin C prevents infection, although high doses during a cold are thought to act as an anti-oxidant and dampen down inflammation.

Other than complete isolation from the rest of the human race, there is no way to avoid catching cold, although keeping your hands washed helps, and. eating well and avoiding stress will strengthen the immune system. Contrary to popular belief, exposure to cold and wet does not increase the risk of infection. Sunlight is thought to kill off cold viruses - which probably explains why we "catch" more of them in winter.

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