A common additive in cosmetics, soaps and toothpastes could be seriously harming your "happy" gut bacteria... or it could be doing absolutely nothing.

Those are the contradictory results of two recently published studies — one in the journal PLOS One, the other in the journal mSphere — looking at the additive triclosan.

CBC Radio science columnist Torah Kachur tries to make sense of the contradiction.

What is triclosan?

This additive is a microbicide — it kills bacteria. But it does so in a different way than antibiotics. It isn't quite as effective, but it works in cosmetic products like soaps. In fact, our most common route of exposure is through toothpaste, particularly the Colgate Total brand. There have been studies that suggest triclosan can reduce gingivitis.

Liquid Soap Safety

Triclosan is a germ-killing ingredient found in many anti-bacterial liquid soaps, as well as toothpaste. Two new studies seem to offer contradictory evidence on whether it's potentially harmful, however. (Kiichiro Sato/Associated Press)

But there have been ongoing questions about its effect on the body — from whether it causes weight gain, to more specific effects like allergies. That's because it could potentially affect our microbes.

We're constantly discovering that microbial ecosystems that live all over us are really important to our health and well-being.

So any agent that seems to disrupt this healthy balance could be problematic. And triclosan is designed to kill microbes.

What did the PLOS One study find?

This was a study from the University of Oregon. It found that triclosan could be harming the microbes that live in guts of animals.

Dawn Antibacterial dish soap

A study suggests triclosan, which can end up in water, has an effect on the gut microbes of zebrafish. (Evan Mitsui/CBC)

The researchers found that when zebrafish were exposed to a triclosan-laden diet, there was a quick change to the microbes that lived in their guts. Within just seven days of being fed triclosan, there was a drastic shift in the types of microbes found there.

How did that affect the fish?

In terms of overall effects, it's still unclear. But there was definitely alteration in how the bacterial community was structured.

Zebrafish, of course, are not humans — but researchers say zebrafish are widely used to study environmental chemical exposures, and that these findings certainly merit further research into the effects of triclosan on the human microbiome.

What did the mSphere study show?

This is where things get interesting. In a second study — published on the same day as the University of Oregon study — researchers from Stanford and Cornell found no appreciable difference in the microbiome in human participants exposed to triclosan compared to those who weren't.

Stanford University's Dr. Julie Parsonnet

Dr. Julie Parsonnet lead a study that found no appreciable difference in the microbiome in people exposed to triclosan compared to those who weren't. (Stanford University)

They measured this by giving participants household and personal care products containing triclosan for four months, and then having them switch to products without triclosan for four months.

"When we looked at the effects of the triclosan-containing products, we really didn't see all that much," said Julie Parsonnet, the lead researcher on the project. 

"We didn't see any major shifts in the microbial flora."

So that seems to contradict the other study, which showed really drastic and rapid effects from triclosan exposure in the zebrafish.

Why were there such different results?

It's really all about dosage. The amount of triclosan that actually stays in your body from a bit of tooth brushing will be negligible.  At least, according to this work, it wouldn't be enough to be cause for concern. And we generally don't go around drinking liquid soap, so that exposure will be small too. 

It's really hard to quantify how much triclosan you are really ingesting or being exposed to — they usually sample urine to figure out the clearance amounts to estimate how much the original dose was.

But dosage matters. If we were exposed to a lot more triclosan, then perhaps it would start to have more negative consequences to our healthy microbes, in the same way that antibiotics do.

Liquid Soap Safety

A new study suggests the bacteria-killing additive triclosan may not be ingested in large enough doses by humans to have negative consequences to our healthy microbes. (Kiichiro Sato/Associated Press)

Antibiotics are more like a carpet bomb of all microbes — good and bad. They aren't just killing the ones making you sick. 

The same goes for the microbicides like triclosan, but it seems like there's not enough dosage to worry.

Should we worry about triclosan exposure?

The book isn't closed yet on triclosan. Although many studies are showing that it may not do significant damage, it really needs to be shown to be beneficial so that its use, cost and exposure levels can be justified.

The next step for the Stanford researchers is to see if there's any correlation between triclosan exposure and weight gain, because another possible consequence of triclosan — something seen in pregnant rats at high doses — is that it could disrupt hormones that lead to the feeling of being full.

At this point in the research, I'm torn on whether to give up my Colgate Total. I tend to veer on the side of being a microbe lover, and try to avoid excess antibiotics or exposure to  microbicides.

But I do really like my toothpaste.