Ranibizumab (Lucentis, Novartis) belongs to a class of drugs that blocks the action of vascular endothelial growth factor A (VEGF‑A). In diabetic macular oedema, VEGF‑A causes blood vessels to leak in the macula, the area of the retina responsible for the clearest vision. The accumulated fluid causes swelling, or oedema, which impairs vision. By inhibiting the action of VEGF‑A, ranibizumab reduces oedema and limits visual loss or improves vision. Ranibizumab has a marketing authorisation for 'the treatment of visual impairment due to diabetic macular oedema in adults'.