A new study has revealed that probiotics aren't always good for you and can in fact be worse if you're taking antibiotics.
Scientists say probiotics should be regulated at the same level as drugs, because the wrong kind of bacteria can actually harm you.
Probiotics are living bacteria and millions of people buy them to help their digestive health.
Microbiologists say they've evolved from containing one kind of bacteria, to containing several different ones.
"The major change would be that the original probiotics were fermented milk products and that goes back about a hundred years," said University of Otago microbiologist Professor Gerald Tannock.
But a new study shows that taking probiotics won't necessarily improve your health and probiotics just pass right through many people's digestive tracts.
It also shows that taking probiotics to counterbalance antibiotics could delay the return of normal gut bacteria.
Scientists say probiotics need to be controlled under law just other drugs, but more research needs to be done.
"If it's true that it's a bad idea to take probiotics with or after antibiotic treatment then you know, that has ramifications commercially and so on," Prof Tannock said.
He told Newshub some trials have shown that certain strains of the good bacteria have positive effects.
But the intake should be tailored to the individual.
"There was an impact in children with eczema and those children had been consuming a probiotics for two years. Other studies show absolutely no effect at all," Prof Tannock said.
Fonterra's senior research scientist James Dekker said there are many other benefits to eating healthy bacteria-rich foods like probiotic yoghurt.
"Fonterra has commercialised two probiotic strains," he said. "We have a strain in our Symbio yoghurt which has shown to be quite effective in areas of gut comfort and reducing gut irritation."
Scientists say the research is in its early days they need time and funding to identify the thousands of different bacterial species in the gut.