The cause of lichen planus is unknown. However, it is thought to be related to either the immune system (the body's natural defence against infection and illness) or an allergic reaction to certain medicines.

Lichen planus is not infectious, does not usually run in families and cannot be passed on to others.

Autoimmune reaction

The immune system protects your body against infection by producing antibodies (proteins) that attack bacteria and viruses.

In people with lichen planus, it is thought that the immune system becomes overactive, causing an excess amount of proteins to be produced which then inflames the skin. This is known as an autoimmune reaction and causes the symptoms associated with the condition.

Reaction to medication

An allergic reaction to certain medications is another possible cause of lichen planus. For example:

• gold injections – these are disease-modifying anti-rheumatic drugs (DMARDs) which are sometimes injected into the muscles of people with arthritis to help reduce swelling (inflammation) and painful joints
• antimalarial tablets – these help to treat and prevent malaria (a tropical disease spread by infected mosquitoes)