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It's possible to have bad breath without knowing. That's because you get used to the smell of your own body. Most people are too polite or embarrassed to comment on bad breath, so you may need to rely on a family member or close friend to tell you if you have it.

Not all bad breath smells the same. How your breath smells may depend on the cause of the smell. For example:¹

• A fruity smell can be a sign of a dangerous condition called ketoacidosis. This can happen if you have diabetes. It means that your body can't use glucose for energy and has to burn fat instead. You should see a doctor right away if you notice this
• A smell like ammonia (a fishy smell) can happen if you have kidney failure
• A smell like feces (stools) can happen if you've been vomiting for a long time.

As you can't smell your own breath, the best way to find out if you have bad breath is to ask a close friend or a relative. Or you can go to see your dentist or dental hygienist.

If you're by yourself, you can get a rough idea of whether your breath smells bad by licking your wrist. Wait a few seconds for it to dry and then smell your wrist. That tells you how the end of your tongue smells.

You can check how the back of your tongue smells by scraping the very back part of your tongue with a spoon, then smelling the stuff on the spoon. That is probably the way your breath smells to others. But be careful about putting a spoon in the back of your mouth. It could make you vomit.

If you go to see your dentist, they can check your breath using one of these tests.²

This is the easiest test. Someone just smells your breath. This test is sometimes done by breathing out slowly through a long tube. Your breath is sniffed as it comes out of the other end of the tube by an examiner, who may be sitting behind a screen.

A dentist can also measure how your tongue smells, by scraping the surface of your tongue with a plastic spoon. They can measure any smell from plaque in your mouth by passing floss between your back teeth.³

To make the results of the test as accurate as possible, you will need to avoid:

• Taking antibiotics. These affect the bacteria in your mouth. But don't stop taking antibiotics just to do this test
• Eating strong-smelling foods, such as garlic or onions, for 48 hours before the test
• Using scented cosmetics, perfume or aftershave for 24 hours beforehand
• Eating, drinking or smoking for 12 hours beforehand
• Brushing, flossing or rinsing your teeth for 12 hours beforehand.

To be sure you really have bad breath, the test should be done on two or three different days if possible.

The person examining your breath can get a better idea of what might be causing your bad breath by comparing the odor coming from your mouth and nose.⁴

• A bad smell coming from your mouth but not your nose is likely to mean a problem in your mouth or throat.
• A bad smell coming from your nose but not your mouth may mean a problem in your nasal passages or sinuses.
• A bad smell coming equally from your nose and mouth may be a sign of a more serious illness, such as liver or kidney problems.

An electric device can test the air from your mouth for chemicals. One of these devices is called a Halimeter. It looks a bit like a car radio with a tube coming out of the front.

As well as testing your breath, your dentist should do a thorough check of your mouth, including your teeth, gums and tongue.³

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.