The main reason trabeculectomy surgery fails is that the drainage site can scar and therefore close up. Antimetabolites, or medications that prevent scarring, can be used to increase the success rate of trabeculectomy surgery in some patients. Since antimetabolites can increase the complication rate of surgery, it is generally reserved for patients that may be at increased risk for trabeculectomy failure. For example, patients who have failed previous trabeculectomy surgery, patients who are younger, patients who have had previous eye surgery in general, patients with uveitis, etc. The decision to use antimetabolites is determined by the surgeon. Not all patients who have trabeculectomy surgery need antimetabolites.