Really, it depends who you ask. Yesterday, a team of researchers from an American university claimed that high-dose injections of it might slow the growth of cancerous tumours. But before oncologists start feeling obsolete, note that this research was carried out on mice. When the mice mainlined the vitamin, tumours in the brain, ovaries and pancreas halved in weight and growth.

But as Dr Alison Ross, science information officer at Cancer Research UK, points out, there is currently no evidence from clinical trials in humans that injecting or consuming vitamin C, or other antioxidants, is effective in treating cancer. "Some research even suggests that high doses of antioxidants can make cancer treatment less effective, reducing the benefits of radiotherapy and chemotherapy," she says.

Along with echinacea, vitamin C is either heroically good for you or ineffectual, depending on the study. Scientists have even claimed that it can kill you. A University of Southern California study in 2000, terrified people into believing that taking very high doses might speed up the clogging of neck arteries, and hasten a heart attack or stroke.

The authorities on such matters are arguably the scientists behind Cochrane reviews, who analyse every published paper on a given topic, then weigh up the evidence. In May last year, Cochrane looked into whether taking vitamin C could cure, or prevent, the common cold. After trawling through 30 trials involving 11,350 subjects, they concluded that regular ingestion of vitamin C has no effect on common-cold incidence in the ordinary population.

It also found insufficient evidence to determine whether vitamin C could do many of the things its believers swear it can, including preventing or treating pneumonia, tetanus, asthma or preeclampsia in pregnancy.