Research shows 93 percent of elite New Zealand athletes are risking their careers by using dietary supplements that could contain illegal substances.
The results of a survey of athletes have been published in a paper in the New Zealand Medical Journal.
The survey found 93 percent of elite New Zealand athletes are consuming the supplements, which are usually some form of vitamin, herbs or amino acids.
It shows many buy them online - 15 percent of which are contaminated with steroids.
The government's anti-doping body, Drug Free Sport, said it was an increasing problem prevalent in many sports including rugby and athletics.
Its chief executive Graeme Steel said athletes who used untrusted dietary supplements were risking long suspensions.
"Probably the majority of the positive tests we've had over a significant number of years has been attributed to inadvertent doping," he said.
"The athletes in many cases have been quite reckless in using the supplements."
Otago University professor of sports medicine David Gerrard, a co-author of the paper, said athletes needed to be more careful.
"There is often a misrepresentation of what is in [the supplements] and that is a risk that athletes must consider," he said.
"Speaking as a physician, most of these supplements are very, very unscientific and there's not a lot of evidence to support their use."
In 2011, International Olympic Committee medical committee chairman Arne Ljungqvist said athletes should not be able to explain away positive tests by blaming them on dietary supplements.
He said it was an unregulated market and athletes should contact their doctor if they felt they needed supplements, as it was the only way to be sure that they did not fall foul of anti-doping legislation.
The research also showed New Zealand had far fewer doping violations than others, with only a handful each year - less than 1 percent of tests.
But Mr Steel said the reality was likely worse.
"The number of those doping who are competing nationally is well under 10 percent, but internationally - well over 30 percent - that's from asking our elite athletes," he said.
"Even if it's 7 or 8 percent in New Zealand, that's of massive concern."
