The aim of this work was to improve the dissolution rate of a poorly water-soluble antibiotic drug, sulfamethoxazole (SMX), by precipitation and co-precipitation with poly(vinylpyrrolidone) (PVP) using the gas anti-solvent (GAS) process. In the precipitation study, the effects of solvent type (acetone, methanol and ethanol), temperature (35, 40, and 45 degrees C) and percent drug saturation (25, 50 and 75%) on the particle size were investigated using the Box-Behnken design of experiment. It was found that an increase in temperature resulted in a reduction in particle size. Moreover, smaller precipitates were produced when using ethanol as a solvent. An increase in percent saturation of the drug in acetone yielded larger particle size. It was also found that after passing through a 200 mesh sieve the precipitates obtained from the GAS process exhibited a higher dissolution rate than the micronized starting material. In the co-precipitation study, it was found that when using a mass ratio of drug and PVP polymer of 1:1, at 50% drug saturation in methanol and 35 degrees C, the highest % drug content (50.0%) was achieved. The dissolution rate of the prepared composites was found to be 10 times greater than that of the GAS precipitates. (C) 2013 Elsevier B.V. All rights reserved.