There are several possible effects. 1) The surface are per amount of drug is increased. This results in faster dissolution rates. In addition, there is more surface energy per amount of material, which encourages aggregation of nanoparticles. Second, the physical properties of the drug can change (crystalline structure, etc.)

Third, for VERY small particles or portions of surfaces with single digit nanometer radii of curvature, the apparent solubility increases according to the Kelvin equation. Finally, again if the particles are VERY small, the molecular packing can be affected, which will change the interfacial tension and lattice energies (assuming their macroscopic properties still hold). The relation between surface energy and lattice energy is not well explored, but has been predicted by the Scapski-Turnbull rule.