Gargling with mouthwash that contains the common antiseptic povidone iodine may reduce the number of coronavirus particles in a patient’s nose and throat, research suggests.
Povidone iodine concentrations as low as 0.5% have inactivated the coronavirus in just 15 seconds in laboratory studies.
To better understand the potential of mouthwashes with this key ingredient, scientists from the University Hospital of Poitiers in France analysed 24 people who had tested positive for the infection in the past 48 hours.
Half the patients – who had coronavirus symptoms but were not severely ill – gargled 25ml of a mouthwash containing 1% povidone iodine four times in a row for five days. They also sprayed 2.5ml of the same solution up each nostril.
After one day, swabs revealed the patients’ viral loads were 75% lower; compared to a 32% reduction among those who did not have the mouthwash treatment.
Although unclear, a reduction in coronavirus particles – known as the viral load – in a patient’s nose and throat may help cut transmission when they sneeze or cough.
Iodine’s medical potential is not new.
In the 4th century, Aristotle’s pupil Theophrastu recorded iodine-rich seaweed could treat sunburn.
It was later used to treat battle wounds during the American civil war. Microbiologist Alexander Fleming, who discovered penicillin, also found iodine effectively prevented gangrene in the first world war.
By the 1960s, iodine was made safer by bonding it with other chemicals, creating povidone iodine. A 1997 study in the journal Dermatology called this “the most potent antiseptic available”.
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In the Poitiers study, the patients were told how to gargle the mouthwash and administer the nasal spray at home.
They also dabbed a 10% povidone iodine ointment up each nostril, which was “massaged” to help the solution spread.
All the patients experienced nasal tingling, however, it was not sufficiently uncomfortable for any to drop out of the study.
Perhaps more alarmingly, their thyroid-stimulating hormone (TSH) levels rose from an average of 2.1 milli-international units per litre (mIU/L) at the start of the study to 3.4mIU/L on day five. For a healthy person, 0.4mIU/L to 4.0mIU/L is considered a normal value.
An international unit is an amount of a substance agreed upon by scientists and doctors. A milli-international unit is one-thousandth of an international unit.
Elevated TSH levels are a sign of an underactive thyroid, with TSH telling the gland to produce more of the hormones that help regulate metabolism.
In five of the patients, TSH levels “exceeded the upper normal value”. These returned to normal between seven and 12 days later, however.
Although generally considered very safe, povidone iodine has been linked to thyroid dysfunction. Anyone with a history of thyroid disease was therefore excluded from the study.
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The scientists acknowledged their trial was small.
A larger study should therefore look into whether povidone iodine “limits the excretion and resulting human-to-human transmission” of the coronavirus, they wrote in the journal JAMA Otolaryngology – Head & Neck Surgery.
Lower povidone iodine concentrations could be tested to reduce side effects, added the team.
It is worth noting, however, the average age of the patients who received the mouthwash treatment was 33, compared to 57 in the control group.
Professor Martin Addy from the University of Bristol called this “highly relevant in a disease where increasing age is a factor”.
“In addition, there is no control intervention which is common practice,” he said.
“The investigators should have used placebo solutions, for instance water as a rinse and nasal infusion.
“There is a well-known and well-studied placebo response, including from my own research, which can be large and in the order of 30%.”
Could mouthwash combat coronavirus transmission?
This is not the first time mouthwash has been discussed as a potential coronavirus treatment.
As well as povidone iodine, mouthwash ingredients like ethanol and cetylpyridinium have been shown to disrupt the fatty membrane of pathogens like the coronavirus.
“In test tube experiments and limited clinical studies, some mouthwashes contain enough of known virucidal ingredients to effectively target lipids in similar viruses,” said Professor Valerie O’Donnell, from Cardiff University.
The coronavirus is known to replicate in the salivary glands and throat, which mouthwashes reach.
It is also said to be “highly sensitive to agents that disrupt lipid bio-membranes”. Other viruses from the same class, which can cause common colds or Severe acute respiratory syndrome (Sars), have been inactivated by “biocidal agents”.
In 2017, a group of scientists found exposing a pathogen from the coronavirus class to 34% ethanol “completely prevented subsequent viral replication”.
When it comes to povidone iodine specifically, Professor Stephen Challacombe from King’s College London previously said: “I have no doubt this should be used and had it been, it would have saved lives.”
Some believe there is nothing to lose, with these mouthwashes being inexpensive, readily available and relatively safe.
Professor Ian Jones from the University of Reading has pointed out, however, it is just a temporary solution, adding “the constant replacement of the virus is the issue”.
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