Some bad breath just can’t be covered up.

Now that dentists have reopened their doors, they’re having patients show up with a nasty set of symptoms, which the doctors have dubbed “mask mouth.”

The new oral hygiene issue — caused by, you guessed it, wearing a mask all the time to prevent the spread of the coronavirus — is leading to all kinds of dental disasters like decaying teeth, receding gum lines and seriously sour breath.

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“We’re seeing inflammation in people’s gums that have been healthy forever, and cavities in people who have never had them before,” says Dr. Rob Ramondi, a dentist and co-founder of One Manhattan Dental. “About 50% of our patients are being impacted by this, [so] we decided to name it ‘mask mouth’ — after ‘meth mouth.’ ”

The term “meth mouth” is widely used by dentists to describe the dental problems that arise among methamphetamine users. Addicts often end up with cracked, black- and brown-stained teeth because the stimulant causes sugar cravings, teeth grinding and jaw clenching. They also often neglect their oral hygiene.

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While mask mouth isn’t quite as obvious, if left untreated, the results could be equally harmful.

“Gum disease — or periodontal disease — will eventually lead to strokes and an increased risk of heart attacks,” says Dr. Marc Sclafani, another co-founder of One Manhattan Dental.

He says the stinky syndrome is triggered by face coverings since wearing a mask increases the dryness of the mouth — and a buildup of bad bacteria.

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“People tend to breathe through their mouth instead of through their nose while wearing a mask,” says Sclafani. “The mouth breathing is causing the dry mouth, which leads to a decrease in saliva — and saliva is what fights the bacteria and cleanses your teeth.”

He adds that “saliva is also what neutralizes acid in the mouth and helps prevent tooth decay and gum disease.”

People’s tendency to drink less water while masked up, as well as consume more coffee and alcohol during lockdown, have also added to the widespread dehydration he’s seeing.

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On the bright side, the dentists say that they’re packing in breath-conscious patients who might otherwise neglect their dental health during the pandemic.

“Patients are coming into us like, ‘Wow, my breath smells, I need a cleaning.’ [But] when you smell the bad breath, you either already have periodontal disease or you have a lot of bacteria that’s sitting on your tongue because of dry mouth,” says Sclafani.

While masks are not negotiable given the times, Sclafani says there are things wearers can do to avoid their grossest side effect: drink more water, cut down on caffeine, snag a humidifier (to “help moisten the air”), use an alcohol-free mouthwash, scrape your tongue and don’t smoke.

And if all else fails? “Just breathe through your nose!”