The power of suggestion is strong enough to make someone sick or even die, according to new research from the United States.

People who think they are going to experience negative side-effects from medicines are more likely to do so, while those who think they will die on the operating table are more likely to do so, according to a team based in Nashville.

Rigorous trials have established over many years that the power of suggestion can have a healing effect, even where no apparently active ingredients are used: the placebo effect.

But the reverse side of the coin is that negative suggestion is equally powerful.

When the "nocebo" effect comes into play, dummy pills and negative expectations can produce harmful effects.

A quarter of patients given placebos during clinical trials -- the control group -- experience side-effects associated with the real version.

For example, patients taking placebos during trials of the blood-pressure medication beta-blockers reported experiencing tiredness and loss of libido just as often as those taking the actual drug.

According to Clifton Meador of the Vanderbilt School of Medicine, negative thinking can actually make you sick, because negative thinking has a profound effect on the body through the emotional components of language and symbolic expression.

Meador said there was nothing mystical about the power of suggestion. However, he said it challenged a strictly biomolecular view of the world.

There are many documented instances of people dying after being cursed, and not just in "primitive" societies.

The pronouncements of the medical profession, with its modern-day trappings of white coats and stethoscopes, can be just as powerful in the developed world as those of a witch doctor in societies which are overtly preoccupied with magic.

Meador said that a cancer patient in the 1970s, Sam Shoeman, died a few months after being told he only had months to live, but an autopsy revealed the tumour had remained small and had not spread as doctors predicted.

At a less dramatic level, many patients who suffer harmful side effects from drugs may do so only because they have been told to expect them.

And people who believe they have a high risk of getting certain diseases are more likely to get them than those who think they are in a low-risk category.