And if the balance gets really out of whack you're opening yourself up to problems like stomach ulcers, bowel problems, bloating and constipation.
However, registered dietitian Amanda Johnson says probiotics need to be taken every single day to get full benefits.
And evidence is still building as to how good they actually are for you, she says.
"If you're feeling good then you probably don't need anything," says Johnson, an expert with 20 years of experience under her belt.
"I don't think there's enough there to say if you have probiotics that you will boost your immune system."
However, they can help people with irritable bowel syndrome and could be worth packing if you're travelling somewhere you're likely to get a funky tummy bug.
Do you take probiotics? Do you have gut problems?
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