The retention of 8 acidic and 2 neutral pharmaceuticals in nanofiltration (NF) was investigated by a loose NF membrane and a tight NF membrane. The retention change due to the exposure to chlorine was studied. The retention of pharmaceuticals was affected by the electric repulsion effect in addition to the size exclusion effect. In the case of the loose NF membrane, the retention of pharmaceuticals was more sensitive to the exposure to chlorine than salt retention because both the decrease in electric repulsion effect and the increase in pore size affected the permeation of pharmaceuticals. On the other hand, in the case of the tight NF membrane, the retention of acidic pharmaceuticals were maintained at higher range at neutral pH range even after exposure to chlorine, though the increase in the pore size was obvious judging from the decrease in the retention of pharmaceuticals in the acidic solution environment.