It is estimated that more than a billion pounds is lost every year to the UK economy due to people taking time off work with severe headache or migraine.

BBC: Family doctors get headache tips

Tension headaches can sometimes be difficult to distinguish from migraines, but unlike some forms of migraine, tension headache usually isn't associated with visual disturbances (blind spots or flashing lights), nausea, vomiting, abdominal pain, weakness or numbness on one side of the body, or slurred speech.

CNN: Tension headache