To investigate the loading of multiple drugs inside single-walled carbon nanotubes (SWCNTs), molecular dynamics simulations were applied to the gemcitabine-SWCNT system with from one to six drug molecules being confined inside the SWCNT cavity. At low drug concentrations (1-4 gemcitabines inside each SWCNT), the pi-pi stacking of two cytosine rings of adjacent gemcitabine molecules caused them to orientate towards each other and move together inside the SWCNT. In contrast, at high drug concentrations (encapsulated gemcitabines >= 5), the close contact of gemcitabines promoted electrostatic interactions through hydrogen bonding rather than the pi-pi stacking interaction from their cytosine rings. (c) 2012 Elsevier B.V. All rights reserved.