Trapped food particles, eating pungent foods and smoking can cause bad breath, as well as gum disease and a dry mouth condition known as xerostomia.

Xerostomia results from hormonal changes, for example, during menopause, as well as from taking some types of medication, said Dr Helena Lee, a peridontist at Mount Elizabeth Medical Centre.

The characteristic stale smell of bad breath arises from sulphur compounds that form in the mouth due to poor oral hygiene, said Associate Professor Lim Lum Peng, a senior consultant in the department of preventive dentistry at the National University Hospital.

"The accumulation of bacteria and substances that form when bacteria decomposes result in the formation of volatile sulphur compounds, which is probably the main source of bad breath," she said.

Both doctors said that medical conditions such as diabetes, lung infections and gastrointestinal problems, can contribute to bad breath too.

What you can do?

Practice good oral hygiene.

Dr Lee recommends brushing teeth twice a day, flossing and using a non-alcohol-based mouthwash.

"Make sure to brush around the gums and the tongue," she added.

Is it worth seeing a doctor?

Yes, said Prof Lim, if the bad breath problem persists despite good oral health care. A doctor will also help exclude possible medical causes.

This article was first published in Mind Your Body, The Straits Times.