The belief that mistletoe can help treat cancer is a myth which can cause harm, warn doctors in this week's Christmas issue of the BMJ.

The warning follows the case of a cancer patient who attended hospital with a tumour-like growth under the skin induced by mistletoe.

Some patients with cancer inject themselves mistletoe extract in the hope of improving their condition, writes complementary medicine expert, Professor Edzard Ernst in an accompanying editorial. In continental Europe, at least 30 different mistletoe preparations are available and 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.

So, is mistletoe an effective anticancer drug and how safe is it?

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, says 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.

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

Yet a wide range of serious adverse reactions have been noted, such as anaphylaxis, breathing difficulties, joint pain and kidney failure. Some studies even suggest that mistletoe extract may enhance the proliferation of some cancers.

So the claim that mistletoe injections have no serious risks is misleading, argues Ernst.

Mistletoe has been tested extensively as a treatment for cancer, but the most reliable trials fail to show benefit, and some reports show considerable potential for harm. The costs of regular mistletoe injections are also high.