Ask for post-exposure prophylaxis (PEP), a morning-after treatment for HIV that may prevent infection. The treatment is a month-long course of HIV (antiretroviral) medications that are most effective if you start them right away—but may still work up to 72 hours after exposure. Side effects can include extreme nausea and fatigue. To find PEP, call a doctor, a health clinic, an AIDS service organization, a health department, or visit your local emergency room.