Using implicit solvent model and replica exchange molecular dynamics, we examine the propensity of a nonsteroidal anti-inflammatory drug, naproxen, to interfere with A beta fibril growth. We also compare the antiaggregation propensity of naproxen with that of ibuprofen. Naproxen's antiaggregation effect is influenced by two factors. Similar to ibuprofen, naproxen destabilizes binding of incoming A beta peptides to the fibril due to direct competition between the ligands and the peptides for the same binding location on the fibril surface (the edge). However, in contrast to ibuprofen, naproxen binding also alters the conformational ensemble of A beta monomers by promoting beta-structure. The second factor weakens naproxen's antiaggregation effect. These findings appear to explain the experimental observations, in which naproxen binds to the A beta fibril with higher affinity than ibuprofen, yet produces weaker antiaggregation action.