What’s that smell? Oh no, it’s my breath. But how can it be so stinky if I just brushed my teeth an hour ago?

If you practice good oral hygiene and haven’t recently smoked a cigarette or bitten directly into an onion, your bad breath might be a sign of an underlying problem.

Infections

If you had a surgery in your mouth — to remove your wisdom teeth, for example — and the wound becomes infected, that could give you smelly breath in the same way tooth decay can. The Sepsis Alliance lists other symptoms of an infection in your mouth as pain, fever, swelling or a bitter taste.

Read: Can You Diagnose Disease By Smelling Someone’s Breath?

An infection in your nose, sinuses or throat may also lead to bad breath because they are connected to post-nasal drip, in which mucus goes from the back of your nose into your throat.

Dieting

The U.K.’s National Health Service warns that your eating habits could lead to bad breath, specifically if you fast or cut out carbs. These diets “cause the body to break down fat, which produces chemicals called ketones that can be smelled on your breath.”

Diabetes

In the same way dieting spurs your body to create those ketones, diabetes is linked to bad breath when it’s poorly managed. When people with diabetes are ill, don’t keep up with their insulin shots, or otherwise don’t have as much of that blood-sugar-regulating hormone as they need, the body is unable to use the sugar you’ve digested as a food source and starts to break down its fat. The ketones are released into the bloodstream in that process, and in large numbers are toxic. Diabetic ketoacidosis can cause fruity-smelling breath as well as rapid breathing, dry mouth, headache, nausea, stomach pain, muscle stiffness and aches. It could make fluid build up in the brain and stop your heart or kidneys, so it is fatal if not treated.

Dry mouth

You may think that saliva is contributing to your bad breath, but actually your saliva is fighting your stinky air. According to the Mayo Clinic, saliva cleanses your mouth and removes the little things that cause bad breath — the trouble comes when your body doesn’t make enough.

“Dry mouth naturally occurs during sleep, leading to ‘morning breath,’ and it worsens if you sleep with your mouth open,” the Mayo Clinic says. “Chronic dry mouth can be caused by a problem with your salivary glands and some diseases.”

“Why you going to the airport? Flying somewhere?” Dumb and Dumber

Medications

There are certain medications that have bad breath as a side effect, according to the National Health Service. Those include nitrates, which are taken for chest pain in cardiac patients; certain chemotherapy drugs; and some tranquilizers.

Tonsil stones

Have you ever felt something hard at the back of your mouth? It might be a tonsil stone, a usually harmless formation that can cause bad breath. They form in the pockets of your tonsils when those little crevices collect debris like mucus and food. The debris calcifies as your white blood cells work them over to protect you from anything dangerous, and may become stuck. If they are highly irritating, an ear, nose and throat specialist may be able to help.

She said she’d never let go, then she killed him with her bad breath. Titanic

Acid reflux

The stuff in your esophagus is only supposed to go in one direction, but some people get acid reflux, in which stomach acid backs up. That could lead to heartburn and bad breath, and in some cases also cause trouble swallowing and nausea that leads to weight loss from a decreased appetite or difficulty eating.

See also:

Avocado for Bad Breath

Office Plants Keep Your Breathing Fresh