A present there is no safe treatment for reducing rate at which disability worsens in people with secondary progressive multiple sclerosis. Recent research has suggested the possibility that drugs that act by blocking the entry of sodium into nerve cells can protect nerve fibres in the brain and spinal cord. In this trial, the investigators will test whether one such drug, called lamotrigine, can prevent damage to nerve fibres and reduce the rate at which MS worsens. The period of treatment in the trial will run for 2 years.