Cyclosporin is a potent immunosuppressant drug commonly used to prevent organ transplant rejection. In recent years, there has been a widening of its therapeutic use and an increase in the number of patients undergoing transplantation. Gingival overgrowth is one of several oral side-effects of cyclosporin, with a quoted prevalence of between 8% and 100%. There is continued debate over the factors which modify the degree of overgrowth, including individual sensitivity, age, dose of drug, duration of drug therapy and the presence of dental plaque. The exact mechanism of gingival overgrowth is still being debated, but appears to be caused by a combination of the proliferation of fibroblasts within the gingival tissue, an increase in the deposition of collagen and extracellular matrix, and a decrease in phagocytosis with a net gain in gingival tissue mass. A number of treatment options are utilized in the treatment of gingival overgrowth, including CO2 laser surgery, improved oral hygiene, the use of antibiotics such as metronidazole and azithromycin, and surgical intervention. In the clinical application of cyclosporin, there is little correlation between cyclosporin dose, serum trough levels and total exposure to the drug, making it difficult to achieve the desired therapeutic response. These problems were previously further complicated by the variability of absorption of the drug via the gastrointestinal tract. The original cyclosporin formulation, Sandimmune, was replaced by a new formulation, Neoral, which has a more reliable absorption, and gives a closer correlation between trough concentration levels and individual bioavailability. There is a conflict of opinion over whether or not the side-effect profile of Neoral varies from its precursor Sandimmune. It has yet to be seen whether the increased bioavailability of Neoral will result in an increased severity and prevalence of gingival overgrowth. An alternative immunosuppressant drug, tacrolimus, which is a macrolide antibiotic with a different side-effect profile, has emerged as a substitute for cyclosporin in organ transplantation. However, there have been conflicting reports of its side-effects and its capacity to cause gingival overgrowth.