Multicentre Randomised Clinical Trial Comparing Fixed vs Pro re Nata (PRN) Dosing of 700 μg Dexamethasone

Inclusion Criteria: 1. Subjects of either sex aged 18 years or over 2. Diagnosis of diabetes mellitus (type 1 or type 2). 3. Best corrected visual acuity in the study eye between ≥34 and ≤73 ETDRS letters at 1m at baseline attributable to diabetic macular edema (DME) 4. On clinical exam at baseline in the study eye, retinal thickening due to diabetic macular oedema involving the centre of the macula and OCT central subfield \> 300 microns despite previous therapy. 5. Media clarity, pupillary dilation, and subject cooperation sufficient for adequate fundus photographs. 6. Ability to return for study visits 7. Visual acuity in fellow eye ≥ 2/60 8. Ability to give informed consent throughout the duration of the study Main Exclusion Criteria: 1. Macular ischaemia 2. Macular oedema is considered to be due to a cause other than diabetic macular oedema. 3. Co-existent ocular disease 4. An ocular condition is present (other than diabetes) that, in the opinion of the investigator, might affect macular oedema or alter visual acuity during the course of the study. 5. A substantial cataract that, in the opinion of the investigator, is likely to be decreasing visual acuity by 3 lines or more.