Non-steroidal anti-inflammatory drugs (NSAIDs) work by interfering with particular enzymes in your body.

Enzymes are a special type of protein which help to bring about, or speed up, a chemical reaction within your body. For example, digestive enzymes help your digestive system break up large food particles into smaller pieces so your body can absorb them.

There are many enzyme systems in the body. One enzyme system involved in inflammation and pain is called the cyclo-oxygenase system.

Cyclo-oxygenase

NSAIDs interfere with an enzyme called cyclo-oxygenase (COX). Different parts of your body have different types of COX enzymes, which control the production of chemicals called prostaglandins. Different prostaglandins have different functions.

For example, the stomach contains a sub-type of COC called COX. This enzyme controls the production of substances called prostaglandins that help protect the stomach from acid (which is normally present in the stomach). Another sub-type called COX-2 enzymes are mainly activated during infection, injury or inflammation, and produce the prostaglandins involved in pain and inflammation.

NSAIDs can reduce pain and inflammation by preventing COX enzymes releasing the prostaglandin chemicals that cause pain and inflammation. However, prostaglandins carry out many different functions within your body. Interfering with them can cause a number of side effects, such as indigestion, inflammation of the stomach lining and stomach ulcers.

COX-2 inhibitors

To prevent side effects, researchers developed a new 'family' of NSAIDs known as COX-2 inhibitors.

These are designed to block the enzymes that cause pain and inflammation but avoid blocking the enzymes that protect the stomach lining. This helps reduce the risk of indigestion and stomach ulcers.

Although COX-2 inhibitors have less effect on the stomach, they might be more likely than traditional NSAIDs to cause side effects to the heart. Further research into this issue is ongoing. This means they may be more suitable for someone at risk of developing stomach or intestinal problems, but less suitable for those with a heart or circulation problem.

Pain

Pain is an unpleasant physical or emotional feeling that your body produces as a warning sign it has been damaged.

Swelling

Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.

Tissue

Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.