How does mouthwash kill Covid-19?

What began as unfounded social media claims could have some truth – but only in certain brands

A health worker takes a swab sample from a tourist coming from Mexico on his arrival at the Jose Marti International Airport amid concerns about the spread of the coronavirus disease, in Havana, Cuba. Reuters

A health worker collects a sample for a Covid-19 test from a baby who arrived in a flight from Mexico at the Jose Marti International Airport in Havana, Cuba. AP Photo

A city worker wearing mask to help protect against the spread of coronavirus, cleans a sidewalk, in Ankara, Turkey. AP Photo

A street musician plays for passers-by in Madrid, Spain, amid the Covid-19 pandemic. AP Photo

A man walks across Lisbon's main Liberdade Avenue. The Portuguese government has put in place safety measures against the spread of the pandemic. AP Photo

An employee sanitises a Hindu temple in Mumbai, India. India is second in the world in total reported coronavirus cases behind the US, but daily infections have been on the decline since mid-September. AP Photo

An employee sanitises a cinema in Mumbai, India. India is second in the world in total reported coronavirus cases behind the US, but daily infections have been on the decline since mid-September. AP Photo

A catholic worshipper holds a sign reading 'Our Lady of Fatima, pray for us' as he takes part with others in a rally in Strasbourg, eastern France, held to protest against a ban on the celebration of masses because of the coronavirus restrictions. AFP

The Washington state and POW flags fly near a red traffic light and a road closed barricade, at the Capitol in Olympia, Washington. AP Photo

A Catholic believer holds a sign reading 'Allow us to pray' as she takes part in a rally in Strasbourg, eastern France, against a ban on the celebration of masses due to coronavirus restrictions. AFP

The Regent Street Christmas lights entitled 'The Spirit of Christmas' stand illuminated after being switched on without a formal ceremony amid England's second coronavirus lockdown, in London. AP Photo

Pastor Regan King, centre, of the Angel church in north London holds a service in near-by Myddelton Square Gardens after striking a deal with the police to hold the service outside. AFP

The Ballard Farmers Market, seen shortly after Washington Governor Jay Inslee's executive orders to combat Covid-19 in Seattle's Ballard neighbourhood. At right is San Fermo, an Italian restaurant's outdoor seating geodome. AP

Tables and chairs from a closed restaurant terrace are piled in a deserted pedestrian street in Lisbon. AP Photo

Army soldiers patrol an empty promenade towards Castel dell'Ovo castle, in Naples, southern Italy, on the first day after being declared a red zone. AP Photo

A woman prays during the Sunday mass at the Church of St Joseph in Paluse, about 120 kilometres north of the capital Vilnius, Lithuania. AP Photo

A man wearing a face mask to protect against coronavirus, walks past a closed shop in Stirling, Scotland. AP

A major motorway linking the capital Beirut to northern Lebanon, is mostly empty of cars during a two-week lockdown to limit the spread of the coronavirus in Dbayeh, Lebanon. AP Photo

An empty street during the new lockdown in Beirut, Lebanon. Lebanese authorities imposed the lockdown due to the rise of Covid-19 cases in the country. EPA

While the world waits for the first Covid vaccines to be distributed, lab tests show that the virus can be killed by some supermarket mouthwashes.

This opens up the possibility that mouth-washing could help to slow the spread of the virus.

But experts said there is as yet no compelling evidence from human studies, and that mouthwashes are not a cure for those with the disease.

What is the evidence that mouthwashes can kill the virus?

Several independent teams of researchers in Germany and the US reported that some over-the-counter products rapidly destroy the Covid virus. Now a team at Cardiff University has published preliminary findings supporting the idea that using mouthwash might provide additional protection against infection.

In laboratory tests designed to mimic conditions in the mouth, the team found that three products – Dentyl Dual Action, Dentyl Fresh Protect and Listerine Advanced Gum Treatment – effectively eliminated the virus. The first two contain the antiseptic cetylpyridinium chloride (CPC), which the researchers believe shows “promising signs” of being able to destroy the Covid virus.

These products join others that have shown encouraging results in lab tests (see table)

However, the Cardiff researchers emphasised that the laboratory findings need to be confirmed in human trials.

But is gargling not already known to be useless?

Early in the pandemic, claims appeared on social media that gargling could cure people infected with Covid-19. Experts pointed out that infection involves the virus entering cells deep in the throat and lungs – beyond the reach of such simple measures.

The new research focuses solely on the possibility that mouthwashes might kill the virus just after it has entered the body, but before serious infection takes hold.

So if it is not a cure, what is the point ?

Slowing the spread of the Covid virus is critical if the pandemic is to be controlled without destroying the economy. If confirmed, the new findings open up the prospect of mouth-washing acting like a “chemical mask”, to be used in addition to standard masks, social distancing and hand-washing.

Some researchers believe slowing the spread of the virus also requires nasal washing. Two compounds already identified as potentially effective as a nasal spray against Covid-19 are 0.5 per cent povidone iodine solution and 1 per cent Johnson’s baby shampoo.

When will we see the results of human trials?

At least half a dozen are already under way, but they all involve small numbers of patients, and so are unlikely to produce compelling evidence.

Very early results from some studies suggest that mouth-washing may be effective outside lab conditions; but the results are inconsistent. They also fail to resolve key questions such as the precise concentrations needed and how long the mouthwashes remain effective once in the mouth.

Why is it all taking so long?

Evidence for the effectiveness of mouth-washing against respiratory diseases such as Covid-19 has been circulating for at least a decade. Listerine was identified as potentially useful during the Spanish flu pandemic a century ago.

However, the studies are generally small and poor quality, and have been widely dismissed or mis-interpreted.

Some researchers also suspect the mouth-washing idea has suffered because it lacks the hi-tech glamour of vaccines – despite its promise as a simple and low-cost means of slowing the spread of the virus.