The belief that mistletoe can help treat cancer is a myth that can cause harm, doctors have warned.

Writing in the British Medical Journal, the doctors highlighted the case of a cancer patient who attended hospital with a tumour-like growth under the skin, that had been induced by mistletoe.

They said that some patients with cancer inject themselves with extract of mistletoe in the hope of improving their condition. In fact, in continental Europe, at least 30 different mistletoe preparations are available, while in Germany, the insurance system pays for this treatment.

A Google search also showed that 145,000 websites promote or mention mistletoe as a treatment for cancer.

The theory that mistletoe might help treat cancer is based on the fact that like cancer, mistletoe is a parasitic growth that eventually kills its host. According to one of the contributors to the BMJ, complementary medicine expert, Prof Edzard Ernst, despite the implausibility of this idea, studies have shown that mistletoe or its main constituents do have anticancer activity.

However many plants have some sort of anticancer activity, but in most cases, the compounds cannot be used, he emphasised.

Nevertheless, supporters claim that regular injections of mistletoe extract improve the natural course of cancer by slowing down or stopping tumour growth and improving quality of life.

However a wide range of serious adverse reactions have been noted, such as breathing difficulties, joint pain and kidney failure. Some studies have even suggested that mistletoe extract may aid the spread of some cancers.

As a result, Prof Ernst said, the claim that mistletoe injections have no serious risks is misleading.

He warned that mistletoe has been tested extensively as a treatment for cancer, but the most reliable trials fail to show any benefits and some reports show considerable potential for harm. Furthermore, the cost of regular mistletoe injections are high.

He therefore recommends mistletoe as a Christmas decoration and for kissing under, but not as an anticancer drug.