Low doses of topically administered vitamin D analogs have been shown to have an anti-psoriatic effect without the risk of hypercalcemia. Calcipotriol, the most thoroughly studied of the vitamin D analogs, was first approved in Europe in the early 1990s. It has been shown to be comparable or slightly more effective than class II corticosteroid ointments. However, patients had reduced levels of parathyroid hormone; mean serum and urine calcium were increased during treatment and hypercalciuria was observed. These effects were reversible with discontinuation of therapy. Thus, while calcipotriol ointment was shown to be effective, the potential for alterations in calcium homeostasis have limited its use to 100 g of ointment per week (0.5 mg calcipotriol/week). Work has continued on the creation of new vitamin D analogs, such as COL-121, with the intent of eliminating the adverse effects of hypercalcemia and hypercalciuria with a compound that is more stable and more easily administered.