Glaucoma is the leading cause of sight impairment and blindness worldwide. It is a long-term eye disease which can cause permanent loss of sight and sometimes blindness and affects 1 in 50 people over 50 years of age. Open-angle glaucoma (OAG) is the most common type of glaucoma. This tends to develop slowly over many years, caused by the drainage pathway in the eye gradually becoming blocked over time due to a build-up of fluid. This build-up causes pressure in the eye to increase (intra-ocular pressure (IOP)), which then damages the important nerve at the back of the eye called the optic nerve, resulting in vision loss. Current treatments offered for glaucoma (eye drops or laser surgery), aim to lower eye pressure and have shown to slow vision loss, however, visual disability and blindness rates remain unacceptably high and many patients continue to lose vision despite these treatments, suggesting that the optic nerve in some patients is more easily damaged. Recent research has looked at cells called 'mitochondria'. These cells produce most of the energy in the body, and the nerve cells in the eye need a lot of energy to function and survive. Nicotinamide (NAM) is a form of Vitamin B3 and evidence so far has shown that mitochondrial function can be improved with this treatment. The aim of this trial is to find out whether taking oral NAM when used with current standard treatment for lowering pressure in the eye, can reduce the amount of sight loss in recently diagnosed patients with OAG, and evaluate the long-term safety and effectiveness of NAM. The trial will use two groups of people recently diagnosed with glaucoma and who have normal care (drops or laser) to lower eye pressure. Using a method of randomisation (randomly allocated to each group using a computer system), one group will be given NAM and the other group will be given a placebo or 'dummy pill'. This is a double masked trial meaning the participant nor the Investigator will be told which treatment group patients have been allocated to.