To assess amiloride, a commonly used water tablet, as a drug that may protect from loss of nerve cells after an episode of Optic Neuritis (ON). ON is an inflammation of the optic nerve (the nerve that carries signals from the eye) and is a common event in Multiple Sclerosis (MS). MS is a disease in which there are episodes of inflammation in either or all of the brain, the spinal cord and the optic nerves (optic neuritis). These episodes generally recover, however following an episode some of the nerve cells are lost. Loss of nerve cells is an important underlying reason for patients with MS to develop disability. In ON this loss of nerve cells can be easily visualised by non-invasive scanning techniques (Optical coherence tomography [OCT] and Scanning laser polarimetry[GDx]) that measure the thickness of the nervous tissue at the back of the eye, known as the retinal nerve fibre layer (RNFL). We know that after an episode of ON, because some nerve cells are lost, this layer th