Disclosed herein is a long acting pharmaceutical composition for treating or preventing human immunodeficiency virus (HIV) infection. The pharmaceutical composition comprises a suspension of a lysine-based aspartyl protease inhibitor or salt thereof, a surface modifier, and a pharmaceutically acceptable carrier; wherein the lysine-based aspartyl protease inhibitor or salt thereof has an average effective particle size of less than about 500 nm. A method for treating or preventing HIV infection with the pharmaceutical composition is also disclosed.