The use of stents has lowered this rate of recurrence or restenosis, and new types of devices that release clot-deterring medicine or even certain forms of radiation have been developed in the hopes of reducing this problem even further. Several investigational clinical studies have been in progress to determine the effective dose levels and optimum delivery systems of gamma or beta radiation (brachytherapy) but the most recent results (five year data) are not encouraging. Both Cordis and Guidant have ceased marketing their brachytherapy devices and many hospitals no longer use them.