The effectiveness of antiretroviral regimes (ARVs) to reduce risk of HIV transmission from mother to child and as post-exposure prophylaxis has been known for almost two decades. Recent research indicates ARVs can also reduce the risk of HIV transmission via sexual intercourse in two other ways. With pre-exposure prophylaxis (PrEP), ARVs are used to reduce risk of HIV acquisition among persons who are HIV negative and significantly exposed to the virus. With treatment as prevention (TasP), ARVs are used to reduce risk of HIV transmission from persons who are already HIV positive. The development of these new prevention strategies raises a rationing problem: given the chronic shortage of ARVs for HIV-infected persons in need of treatment, is it ethically justified to allocate ARVs for PrEP and/or TasP? This article examines the intuitively appealing view that allocation of ARVs for treatment should be the highest priority, the use of ARVs for TasP should be a secondary priority, and that utilizing ARVs for PrEP would be unethical. I will argue that selective, evidence-based allocation of ARVs for prevention in certain cases could be ethically justified even when there is insufficient anti-retroviral access for all those needing it for treatment.