Alka-seltzer original effervescent tablets contain three active ingredients, aspirin, sodium hydrogen carbonate and citric acid. Aspirin belongs to a group of medicines called non-steroidal anti-inflammatory drugs (NSAIDs). It works by blocking the action of a substance in the body called cyclo-oxygenase. Cyclo-oxygenase is involved in the production of various chemicals in the body, some of which are known as prostaglandins. Prostaglandins are produced in response to injury or certain diseases and would otherwise go on to cause pain, swelling and inflammation. Aspirin is therefore used to relieve pain and inflammation. Sodium hydrogen carbonate and citric acid form antacids by their effervescent reaction with water. They decrease the acidity of the stomach's juices, by binding to and neutralising excess acid. This helps to relieve the symptoms of heartburn and indigestion and settles the stomach. The combination of these medicines is used primarily to relieve headaches accompanied by an upset stomach, which can ofter occur after over-indulgence in food, drink or both. It can also be used to relieve various other types of aches and pains.