Most cases of gastro-oesophageal reflux disease (GORD) are thought to be caused by a problem with the lower oesophageal sphincter (LOS).

The LOS is a ring of muscle at the bottom of the oesophagus (gullet), which is the tube that runs from the back of the throat to the stomach.

The LOS works as a valve, opening to let food into your stomach to be digested by stomach acid. It then closes to stop any acid leaking back up into your oesophagus.

However, in people with GORD, this ring of muscle has become weakened, allowing stomach acid to pass back up into the oesophagus. This causes symptoms such as a burning pain in your chest (heartburn), or a feeling of discomfort in your stomach.

It's not always clear what causes the LOS to become weakened, but a number of risk factors have been identified (see below).

Risk factors

Risk factors for a weakened LOS include:

• being overweight or obese – this can place increased pressure on your stomach, which in turn can weaken the LOS
• eating large amounts of fatty foods – the stomach takes longer to dispose of stomach acid after digesting a fatty meal
• consuming tobacco, alcohol, coffee or chocolate – it's been suggested that these four substances may relax the LOS
• being pregnant – changes in hormone levels during pregnancy can weaken the LOS and increase pressure on your stomach
• having a hiatus hernia – this is where part of your stomach pushes up through your diaphragm (the sheet of muscle used for breathing)
• stress 

There's also a condition called gastroparesis, where the stomach takes longer to dispose of stomach acid. This excess acid can push up through the LOS.

Gastroparesis is common in people who have diabetes, because high blood sugar levels can damage the nerves that control the stomach.

Medication

Certain medications can relax the LOS, leading to the symptoms of GORD. These medications include:

• calcium-channel blockers – used to treat high blood pressure (hypertension) 
• nitrates – used to treat angina, which is chest pain caused by the blood supply to the muscles of the heart becoming restricted

There are also a number of medications that can contribute to the development of oesophagitis (where stomach acid irritates and inflames the lining of the oesophagus). These include:

• non-steroidal anti-inflammatory drugs (NSAIDs) – a type of painkiller, including ibuprofen
• selective serotonin reuptake inhibitors (SSRIs) – a type of antidepressant
• corticosteroids (steroid medication) – which are often used to treat severe symptoms of inflammation
• bisphosphonates – used to treat osteoporosis (weakening of the bones)