New University of Otago research which found undeclared anabolic steroids in sports supplements should be a wake-up call for athletes at all levels, Drugfree Sport New Zealand says.
The study tested 116 sports supplements readily available in shops or over the internet.
Six contained androgens - a designer anabolic steroid.
Researchers said the androgens posed a potential health risk for buyers, and professional athletes using the product could unwittingly fail a drugs test.
Labelling and marketing of sports supplements was poorly regulated, meaning products promising unsubstantiated benefits could be widely sold, they added.
The study's findings came as no surprise to Nick Paterson, chief executive of Drugfree Sport New Zealand, a Crown entity responsible for implementing anti-doping codes in New Zealand sports.
''We have been warning about this for some time ...
We know we have people buying supplements from shops in New Zealand, which is one thing: you know who you are buying from and it might be a reputable brand. But we also know we have people buying supplements on the internet from anonymous sources.''
The risk from buying online was magnified, he said.
Anyone considering using sports supplements should ask if they really needed them.
Supplements could include rehydration powders or vitamin supplements.
Although the study focused on steroids not declared on the product packaging, an athlete could be found guilty of a doping violation if they had unwittingly taken then, Mr Paterson said.
''With supplements there is no guarantee that everything will be on the label, and it's very difficult for people to find out.''
The study, carried out by University of Otago and University of Technology Sydney, was published in the International Journal of Sport Nutrition and Exercise Metabolism.
Lead author Prof Alison Heather, of Otago University's Department of Physiology, said European studies had shown up to 16% of sports supplements contained a banned substance.
''Supplements can contain ingredients that may have useful properties,''she said.
''However, due to poor manufacturing practices or adulteration, sports supplements can contain compounds that are banned for use in sports, but ... not on the label as an ingredient.''
A 2015 report showed 95% of elite New Zealand athletes used sports supplements, while a 2014 report suggested 70% of New Zealand First 1st XV schoolboy rugby players regularly took them.
