The present invention is directed to methods for inhibition of HIV reverse transcriptase, treatment of infection by HIV, prophylaxis of infection by HIV, and the treatment, prophylaxis and/or delay in the onset or progression of AIDS or ARC by administering a compound of structural Formula I, or a pharmaceutically acceptable salt or co-crystal thereof, wherein X is -F or -Cl, less frequently than once daily.本發明係關於藉由低於每日投藥一次的頻率投與結構式I之化合物或其醫藥學上可接受之鹽或共晶體來抑制HIV反轉錄酶、治療HIV感染、預防HIV感染,及治療、預防及/或延遲AIDS或ARC之發作或演進的方法, 其中X為-F或-Cl。