5-Fluorouracil (5-FU) is an antimetabolite and is used to improve the success rate of trabeculectomies by inhibition of subconjunctival fibrosis. It can be administered postoperatively as a subconjunctival injection or given intraoperatively via cellulose sponge soaked in 5-FU and placed between the sclera and conjunctival flap. The advantage of injecting 5-FU subconjunctivally postoperatively is that the dose and frequency can be titrated according to patients’ needs. Subconjunctival injection may be uncomfortable and this could be due to the alkaline nature of 5-FU (pH 9.20) and its osmolarity of 384 mmol/l. It would be beneficial if this discomfort can be minimised as the injection may need to be given several times.