The stomach naturally makes acid as part of the normal digestion process. Usually the sphincter (ring of muscle) at the bottom of the oesophagus (the food pipe that connects your mouth with your stomach) acts as a valve and keeps this acid in the stomach. If this sphincter relaxes when it shouldn't, the contents of the stomach can rise up into your oesophagus, and sometimes all the way up to your mouth. Some factors that make acid reflux more likely include: 

• weakness of the oesophageal sphincter
• too much acid made in the stomach
• if the stomach is not emptying properly
• if there is increased pressure in the stomach, forcing acid back up into the oesophagus (for example, during pregnancy)
• certain types of food such as spicy acidic or fried foods
• certain medications such as anti-inflammatories (check the label of your medication for side effects if you're not sure)
• presence of a hiatus hernia (where the upper portion of the stomach bulges up through the diaphragm, into the chest. The diaphragm is a layer of muscle that separates the chest from the abdomen.
• Smoking
• Eating large meals or eating late at night
• certain beverages, such as alcohol or coffee