Bad breath (halitosis) has a number of possible causes.

Poor oral hygiene

The most common cause of bad breath is poor oral hygiene.

Bacteria that build up on a person's teeth, tongue and gums can cause plaque (the soft, white deposit that forms on the teeth's surface), gum disease and tooth decay.

The bacteria combine with saliva to break down food particles and proteins – this releases an unpleasant-smelling gas.

If you don't brush and floss your teeth regularly, any food trapped between your teeth will be broken down by the bacteria, causing bad breath.

Bacteria can also live in the rough surface of your tongue. Therefore, as well as brushing your teeth, cleaning your tongue can also help control bad breath.

Having regular dental check-ups will ensure that any oral hygiene problems are picked up and treated early. Your dentist can advise on how often you need a check-up.

Food and drink

Eating strongly flavoured foods – such as garlic, onions and spices – is likely to make your breath smell. Strong-smelling drinks – such as coffee and alcohol – can also cause bad breath.

Bad breath caused by food and drink is usually temporary, and can be avoided by not eating or drinking these types of food and drink too often. Good dental hygiene will also help.

Smoking

Smoking is another cause of bad breath. As well as making your breath smell, smoking can also stain your teeth, irritate your gums and lessen your sense of taste. 

Smoking also increases your risk of developing gum disease, which is another cause of bad breath. Stopping smoking will lower your risk of gum disease and thus help prevent bad breath.

Crash dieting

Crash dieting, fasting and low-carbohydrate diets can also cause bad breath. These cause the body to break down fat, which produces chemicals called ketones that can be smelt.

Medication

Some types of medication can also cause bad breath. Medications associated with bad breath include:

• nitrates – which are sometimes used to treat angina (chest pain caused by a restriction in the blood supply to the heart)
• some chemotherapy medication
• tranquilisers (phenothiazines)

If the medication you're taking is causing bad breath, your GP may be able to recommend an alternative.

Medical conditions

In rare cases, bad breath can be caused by certain medical conditions.

Dry mouth, also known as xerostomia, affects the flow of saliva. A lack of saliva can cause bacteria to build up in the mouth, leading to bad breath.

Dry mouth can sometimes be caused by salivary gland problems or breathing through your mouth instead of your nose.

In some cases, gastrointestinal conditions may cause bad breath. For example, H. pylori infections (bacterial infections of the stomach lining and small intestine) and gastro-oesophageal reflux disease (GORD) have been linked with bad breath.

If a gastrointestinal condition is thought to be causing your bad breath, you may need to have an endoscopy. This is a procedure where a piece of equipment called an endoscope is used to examine an area inside the body, such as your airways or abdomen.

Other medical conditions that can cause bad breath include:

• lung, throat or nose infections, such as bronchiectasis  
• bronchitis
• sinusitis
• diabetes