A vaginal ring worn for three months may prevent HIV as well as the existing one-month ring, offering greater convenience at a lower cost. The flexible silicone ring slowly releases dapivirine, a non-nucleoside reverse transcriptase inhibitor. A one-month ring is authorized in 11 African countries, but it is not yet approved in the United States. The one-month ring is moderately effective. In one trial, it reduced the risk of HIV acquisition by 27% overall, rising to 56% for older women. A real-world study in Zimbabwe showed that HIV incidence was similar for women who used the ring or daily pre-exposure prophylaxis (PrEP) pills. In a Phase I trial of 124 women in South Africa, a three-month ring containing four times as much dapivirine led to higher drug levels in blood plasma and vaginal fluid. Both versions of the ring were safe and well tolerated. Surveys have shown that when offered a choice of prevention methods, a substantial number of women prefer a vaginal ring over daily oral or long-acting injectable PrEP, and some research suggests that rings may encourage better adherence than pills.