Exactly what causes acid reflux in adults is still the subject of much debate and research.
Acid reflux can develop for a number of different reasons and many different factors can increase the chances of you developing acid reflux disease.
Smoking is a large contributor with acid reflux disease and is known to cause a number of different problems that can develop into GERD (Gastro Esophageal Reflux Disease).
Smoking can damage the mucus layer that protects the stomach lining from the effects of stomach acid. Our swallowing reflex in the esophagus can also be damaged by nicotine making it more difficult to swallow and more likely that food and stomach contents will be regurgitated.
Smokers have increased levels of acid secretions that can worsen the symptoms of acid reflux and heartburn. It can also weaken the LES (Lower Esophageal Sphincter). This is the circle of muscle that seals the top of your stomach keeping your stomach contents in.
When this is weakened it makes it easier for food and stomach acid to be refluxed back into the esophagus, causing heartburn.
Smoking can also reduce saliva production which the body uses to aid digestion and neutralize the acid in our stomach.
It also increases the risk of some types of throat cancer.
Long term acid reflux can lead to changes to the esophageal lining. The lining changes to resemble the lining of the stomach, developing their own protective layer of mucus. This condition, known as Barretts Esophagus can lead to esophageal cancer in a small percentage of sufferers and needs to be regularly monitored.

Abnormalities in the stomach, such as a hiatal hernia can also cause an increased risk of acid reflux disease. Normally, our esophagus passes through the diaphragm to the stomach. In someone who has suffered a hiatal hernia, part of the stomach and the LES gets pushed through and trapped above the diaphragm. Whenever we eat, food can get caught in this portion of the stomach, increasing pressure on the lower esophageal sphincter and causing heartburn.

During pregnancy it is common to experience acid reflux especially in the third trimester when the baby is larger and puts pressure on the stomach. Changes in hormone levels also weaken the LES. These two factors combine, resulting in regular heartburn and acid reflux. Happily the symptoms stop in the vast majority of cases as soon as the baby is born.

Certain foods, commonly known as trigger foods are linked to acid reflux disease and heartburn, although, rather than being the cause of acid reflux they tend to be the triggers that set off symptoms in someone who has already developed acid reflux disease.

Alcoholic drings, fizzy drinks, chocolate, citrus fruits, tea and coffee, fatty and fried foods, tomato based food and tomatoes,garlic and onions, peppermint and spices and spicy food are all believed to worsen acid reflux disease in some people.

Other things that contribute to acid reflux are :
Exercise too soon after eating can lead to acid reflux as it can increase the pressure within the abdomen.
Eating late at night, which can lead to night-time reflux.
Being overweight which puts added pressure on your including the stomach.
Bending at the waist soon after a meal can put pressure on the stomach and cause to acid reflux.
Certain drugs such as aspirin and NSAIDs (non steroidal anti-inflammatory drugs) have also been linked to acid reflux disease.

Fortunately, acid reflux is easily controlled in most people with a mix of drugs to reduce acid levels and lifestyle changes and diet.