Users of vitamin and mineral supplements risk severe poisoning if they take too many capsules, the British food regulator has warned.
In a tough report that for the first time outlines probable overdose levels, it says side effects, ranging from bone weakness and stomach damage to cancer, are linked with such common supplements as vitamins A and C, zinc and iron.
Britain's Food Standards Agency says that in some supplements an overdose can occur with as little as double the daily recommended amount.
The agency has recommended that one popular supplement be withdrawn from sale altogether because of a cancer link.
Chromium picolinate is widely available in Australia in diet pills and as a stand-alone supplement used by body-builders. It was down-graded to "low-risk" status in Australia by the federal Health Department in 1998.
The British report, which calls into question the unsanctioned but common practice of "mega-dosing", is likely to undermine further the beleaguered Australian supplements industry, still reeling from the Pan Pharmaceuticals recall.
About half of all vitamin and mineral supplements have been pulled from retailers' shelves.
The technical director of the Complementary Healthcare Council, Allan Crosthwaite, said the issue of what warnings should appear on supplement labels was under review by the Therapeutic Goods Administration.
"It is being discussed in that forum - how we strengthen the messages to consumers and how we ensure they understand them," he said.
The professor of clinical pharmacology at the University of NSW, Ric Day, welcomed the British warnings.
"There is an issue of people taking more than is recommended on the labels, and that is not a sensible thing to do," he said.
"Some people are less able to handle vitamins than others."
The sick, the elderly and those with liver or kidney problems needed to be especially cautious, Professor Day said. The greater variety of supplements people took, the more likely they were to experience side effects, he said.
An obstetrician and gynaecologist, Alastair MacLennan, of the Women's and Children's Hospital in Adelaide, said doctors frequently encountered the consequences of unwise vitamin use.
"I'm aware of oral contraceptive pill failures from women taking vitamin C, which interferes with oestrogen metabolism," Professor MacLennan said.
For the same reason, post-menopausal women could experience unexpected bleeding if they used vitamin C, which could lead to unnecessary surgical investigations.
Australian recommended daily intakes for a variety of nutrients were established by the National Health and Medical Research Council, mainly during the 1980s. But upper safe levels have not been determined.
Colin Binns, co-chairman of the council's dietary guidelines group, said the recommended intakes would be revised next year, and the group would probably borrow from the British report to fix maximum safe levels.
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