In ads on TV, it all looks so simple. People use mouthwash, it instantly neutralises all the nasty bacteria hiding in their mouths, and – just like that – their dental hygiene is assured.

But what's really going on when you rinse a cap-load of antibacterial chemicals around your mouth? What does that to your body, and to other kinds of microorganisms that may actually be beneficial to health?

As a study showed last year, the downstream effects can be surprising, and far-reaching too, affecting much more than just your dental wellbeing.

In an experiment led by scientists from the UK and Spain, researchers found that the simple act of using mouthwash after exercising can reduce one of the benefits of exercise: lowering blood pressure.

When you exercise, your blood vessels open in response to the production of nitric oxide, which increases the diameter of blood vessels. This process is called vasodilation, and it increases blood flow circulation to active muscles.

For a long time, researchers thought this only happened during exercise, but in more recent years, evidence has shown that circulation stays high (meaning blood pressure is lowered) even after exercise – thanks to how bacteria interact with a compound called nitrate, which forms when nitric oxide degrades.

"Research over the last decade has shown that nitrate can be absorbed in the salivary glands and excreted with saliva in the mouth," explains physiology specialist Raul Bescos from Plymouth University.

"Some species of bacteria in the mouth can use nitrate and convert into nitrite – a very important molecule that can enhance the production of nitric oxide in the body."

Once nitrite is produced and swallowed with saliva, it becomes absorbed into blood circulation and reduces back to nitric oxide, which keeps blood vessels wide and lowers blood pressure.

But according to this small study, it looks like this biological mechanism can be significantly interrupted if anti-bacterial mouthwash gets added into the post-exercise mix.

In an experiment, 23 healthy adults ran on a treadmill for 30 minutes. After the workout, the participants were asked to rinse their mouth with either an antibacterial mouthwash or a mint-flavoured placebo.

These mouth rinses occurred immediately after the exercise, and also at 30, 60, and 90 minutes after.

The participants also had their blood pressure taken during the experiment, immediately after the exercise, and during their rest period.

The results showed that at one hour after the treadmill session, average reduction in systolic blood pressure in the placebo group was –5.2 mmHg (millimetres of mercury).

The reduction in the mouthwash-using group was much lower, showing an average of –2.0 mmHg at the same point in time, suggesting the use of the antibacterial mouthwash (0.2 percent chlorhexidine) had lowered the systolic blood pressure reduction by over 60 percent.

At the end of the monitoring window, two hours after the treadmill session, the mouthwash group showed no sign of blood pressure reduction stemming from the exercise, whereas the placebo group still showed a significant reduction compared to their pre-exercise values.

"This is the first evidence showing that the nitrate-reducing activity of oral bacteria is a key mechanism to induce the acute cardiovascular response to exercise during the recovery period in healthy individuals," the authors explained in their paper.

While it's only a small study, it serves as an important reminder of how not all bacteria are necessarily bad for us – and that ingesting antibacterial chemicals that indiscriminately terminate mouth-dwelling microbes could hamper important biological processes necessary for good health.

"These findings show that nitrite synthesis by oral bacteria is hugely important in kick-starting how our bodies react to exercise over the first period of recovery, promoting lower blood pressure and greater muscle oxygenation," said one of the team, nutritionist Craig Cutler.

"In effect, it's like oral bacteria are the 'key' to opening up the blood vessels. If they are removed, nitrite can't be produced and the vessels remain in their current state."

The findings have been reported in Free Radical Biology and Medicine.

A version of this article was first published in September 2019.