HIV blocking medication is generally recommended to be taken within 72 hours, and is most effective when taken within 2 hours, usually the earlier the better. The so-called HIV blocking medication is a prophylactic medication that is taken after HIV infection, usually in combination, for 28 days. If the HIV test is negative, there is no infection, which means that the blockade is successful. If the HIV test is positive at a later date, the blockade has failed.