The World Health Organization warns the unregulated use of alternative medicines can cause unpleasant or potentially dangerous reactions.
More and more people use herbal medicines
It has issued new guidelines advising consumers on therapies ranging from acupuncture to herbal medicines and food supplements.
They are aimed at helping those who buy complementary medicines over-the-counter and do not tell doctors.
The WHO said such medicines were not "good for everybody all of the time".
As increasing numbers of people in industrialised and developing countries use alternative medicines, there are increasing reports of adverse and even fatal
The WHO said that, although there were no global statistics on reactions to the medicines, individual countries were reporting problems.
It said that in China, there were 9,854 cases of adverse reactions were reported in 2002 alone, more than double the number registered during all of the 1990s.
Xiaorui Zhang, WHO's coordinator for traditional medicines, said that consumers often assumed that "natural means safe", but lacked knowledge about using such products properly.
She added: "Most countries have no regulations to control herbal products. More than 90 countries sell them over-the-counter."
Some countries are taking steps to limit the risk to consumers. In December, the US Food and Drug Administration issued a warning about dietary supplements containing ephedra, also called Ma huang, a natural substance used in China to treat people for coughs.
Sales of ephedra are already restricted in the UK.
'Lack of knowledge'
Vladimir Lephakin, WHO assistant director-general for health technologies and pharmaceuticals, said: "It is not true that good, traditional medicines are good
for everybody, every time in big quantities. This is a big mistake"
He added: "There are a lot of examples of people who not only suffer but die because of drug interaction or non-proper use of traditional medicine."
He said food supplements, which are not often regulated as medicinal
products, also lacked quality controls.
Mr Lephakin added some studies had found that some products in different countries contained toxic heavy metals and in extreme cases there were traces of narcotics to make the products addictive.
He said: "There is a need for strengthening control of food supplements in all countries."
An EU directive, due to come into force on 1 August 2005, will harmonise the rules on vitamins and food supplements across the European Union.
It includes a list of vitamins and minerals which can be used in food supplements. It also includes upper limits on certain vitamins.