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Bad breath, also called halitosis, affects nearly half the general population. Most of the time, it’s caused by something in your mouth or throat. But in some cases, an underlying medical condition can cause a particular mouth odor.

Because bad breath can be an indication of an underlying health issue, it’s important to know when it’s just an annoyance and when you need to see a medical or dental professional.

Sometimes, the type of odor can help you pinpoint the underlying problem.

Bad breath can arise from a problem in your mouth or digestive tract or from metabolic processes taking place in your body.

Let’s take a closer look at some of the most common bad breath smells and the possible causes.

Breath that smells sweet or fruity

Unmanaged diabetes can lead to a dangerous health condition called diabetic ketoacidosis. Among other symptoms, it can cause your breath to smell sweet or fruity.

Low carb diets and fasting can sometimes also cause your breath to smell or taste different. Some people describe it as metallic. For other people, the smell is sweet.

Low carb diets cause you to burn body fat for fuel, which leads to the release of chemicals called ketones in your breath and urine. The buildup of ketones can cause changes to the smell of your breath.

Breath that smells rotten or fetid (like garbage or death)

An abscess or infection in your mouth, throat, or lungs may cause your breath to smell like rotting tissue.

For example, bronchiectasis, a condition that causes your bronchial tubes (air passages) to thicken and widen, can lead to repeated respiratory infections and excess mucus with a strong fetid odor.

Also, when dentures, crowns, and orthodontic devices don’t fit properly, food can become wedged in gaps. The odors from old food and bacterial growth may smell like decay.

Likewise, poor dental health can lead to:

  • cavities
  • ulcers
  • lesions
  • fistulas

These wound-like openings can trap decaying food or emit rotting smells, too.

Another cause could be untreated periodontal disease (gum disease).

Granulomatosis can also cause rotten or decayed-smelling breath. This condition is a rare inflammatory disorder that causes problems with your blood vessels, kidneys, and nose. It’s treatable if detected early, but the condition can become more dangerous if it progresses without treatment.

Breath that smells like nail polish remover

Carbohydrates provide quick-burning fuel for your body. When you’re following a low carb diet like keto or paleo programs, you aren’t consuming very many carbs. As a result, your body burns stored fat instead of carbs, and this can produce a chemical called acetone in the process.

Acetone is the same chemical found in many nail polish removers. Diabetes can also cause the release of acetone.

Breath that smells sour

With gastroesophageal reflux disease (GERD), the muscle between your esophagus and stomach doesn’t close properly. As a result, the contents of your stomach can back up into your esophagus, throat, or mouth.

If you have GERD, your breath may sometimes smell sour, like partially digested food.

Breath that smells like feces

If something is blocking the flow of waste through your intestines, your breath may begin smelling like feces.

With a blockage, you’re likely to experience the following in addition to bad breath:

  • bloating
  • vomiting
  • cramping
  • nausea
  • constipation

If you experience these symptoms, it’s important to get medical attention right away, because a bowel obstruction can be life threatening.

Breath that smells of ammonia or urine

Breath that smells of ammonia or urine is known as azotemia. This condition is typically caused by damage to your kidneys, either through injury or disease.

If your kidneys can’t expel enough nitrogen, chemicals build up inside your body, leading to the ammonia odor.

Breath that smells musty

People with liver disease, including cirrhosis, have breath with a characteristically musty smell.

The distinctive smell, fetor hepaticus, is produced by volatile organic compounds (VOCs) that build up in the body when the liver isn’t functioning properly. Dimethylsulfide is believed to be chiefly responsible for the musty smell.

Breath that smells like maple syrup

The inability to metabolize three types of amino acids (leucine, isoleucine, and valine) can cause maple syrup urine disease, in which a person’s breath or urine smell like maple syrup or caramelized sugar.

This disease can cause serious health and developmental problems if it isn’t diagnosed and treated early.

Breath that smells like sweaty feet

Being able to break down proteins is vital to the healthy functioning of your body. When your body doesn’t produce enough of the right kinds of enzymes to break down amino acids, your breath may take on one of several distinctive odors, depending on what type of enzyme isn’t working properly.

Isovaleric acidemia, a genetic condition that affects infants, causes a buildup of leucine in the blood, leading to an odor some say smells like sweaty feet.

Breath that smells fishy

Trimethylaminuria is another enzyme disorder in which your body can’t break down trimethylamine, an organic compound. This can cause your breath, sweat, and other bodily fluids to exude a fishy smell.

Breath that smells of boiled cabbage

Hypermethioninemia, a genetic disorder, occurs when your body can’t metabolize the amino acid methionine. It causes your breath and urine to take on the smell of boiled cabbage.

People with this condition often don’t have any other symptoms beyond this kind of halitosis.

Bacteria

Your mouth is warm and wet — ideal conditions for breeding bacteria. Over 500 different strains of bacteria can be found in your mouth, mostly on the top surface of your tongue and in the crevices between your teeth.

These bacteria cause the release of volatile sulfuric compounds (VSCs), gasses that make your breath foul.

Smoking

A 2014 studies showed that approximately 80 percent of people who smoke have halitosis. Smoking itself can cause bad breath, and smoking can also cause mouth and gum diseases that worsen breath.

Dry mouth

Saliva breaks down food and helps manage bacterial growth in your mouth. If your salivary glands don’t make enough saliva, your mouth dries out — a condition called xerostomia. Xerostomia can lead to gum disease, cavities, and other odor-causing problems.

Some medications can also dry out your mouth, including:

  • antidepressants
  • antipsychotics
  • diuretics
  • blood pressure medications
  • some cancer medications

In rare cases, a disorder in the salivary gland can interfere with saliva production.

Because bad breath can make you feel self-conscious, there are numerous remedies on the market to correct it.

Here are some steps you can take to eliminate bad breath and keep your mouth healthy.

Practice good dental hygiene

The American Dental Association recommends brushing twice daily for at least 2 minutes. Daily flossing is also important because food particles can get stuck in places a toothbrush can’t easily reach.

If you have dental appliances, a device that uses water to jet away food, like a Waterpik, might work better than floss or tape.

Some mouthwashes kill bacteria as they freshen your breath, and many contain fluoride to protect your teeth and gums from decay.

Get regular dental checkups

Seeing a dentist twice a year is an important preventative measure. Regular cleanings can remove stubborn plaque buildup that can be hard to get rid of on your own.

Your dentist or hygienist can also show you techniques and devices for making sure you follow the most effective at-home dental hygiene practices.

Because bad breath is most often caused by something going on in your mouth or throat, a dentist can help isolate the cause.

Talk to a healthcare professional

If a health condition is causing your bad breath, you’re probably also experiencing other symptoms besides bad breath. Treating the smell of your breath may require getting the medical condition under control.

If one of your medications is causing dry mouth, talk to your healthcare professional about alternatives that won’t cause this problem.

Stop using tobacco products

Smoking cigarettes and using chewing tobacco or snuff can increase your risk of many serious health conditions, including:

  • lung cancer
  • mouth cancer
  • throat cancer
  • periodontal disease

Not only will quitting tobacco improve your breath, but it may also reduce your risk of many other health problems.

Use products that help with dry mouth

It’s important to stay well-hydrated, especially if you often experience dry mouth. You may also want to consider sleeping with a humidifier in your room if your dry mouth tends to be caused from sleeping with your mouth open.

Limiting caffeine, antihistamines, and alcohol may also help. Also, try not to use an oral rinse or mouthwash that contains alcohol.

There are a number of products on the market to moisten your mouth, including rinses, chewing gums, and mouth sprays that can help replenish fluids.

Most bad breath issues are caused by what’s happening in your mouth or throat.

Bacteria, gum disease, cavities, dry mouth, poorly fitting dental devices, and infections are among the leading causes of bad breath. Sometimes problems in the intestinal tract, liver, and kidneys are at the root of the problem.

To prevent or treat bad breath, practice good dental hygiene and be sure to get regular dental checkups. If your dentist can’t pinpoint the cause of your bad breath, talk to your doctor. They can help determine if your breath odor is caused by an underlying health condition.