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The antibiotic cotrimoxazole was associated with poly(lactic acid-co-glycolic acid) (PLGA) and maghemite, aiming to reach a controlled drug release system. PLGA was synthesized through the polycondensation of lactic acid and glycolic acid in an equimolar ratio, and maghemite was synthesized through the coprecipitation method. The drug cotrimoxazole was inserted in the composite through three different procedures: solution, fusion, and in situ to check the best insertion method. Several techniques were used to characterize the materials. The copolymer was characterized by nuclear magnetic resonance and size-exclusion chromatography. In addition, the maghemite, the composites containing the drug, and the polymer were characterized by Fourier transform infrared spectroscopy (FTIR) with attenuated total reflectance device, small-angle X-ray scattering, and magnetic force, this last according to the methodology developed by our group. The root mean square error was used to compare the FTIR spectra of the samples, proving that the fusion method was the best way to insert the drug and maghemite in the polymer. Therefore, composites containing the drug and the nanoparticles were prepared by the fusion method. These composites were used for dissolution profile studies, which were monitored with and without magnetic field, aiming to understand the influence of the magnetic field on the dissolution profile. The dissolution was monitored and quantified using the ultraviolet-visible spectrophotometry, following the United States Pharmacopeia (USP) method for cotrimoxazole tablets. Results demonstrated that nanocomposites presented a good magnetic force, able to keep the magnetic composite trapped in a specific place or tissue. The presence of the nanoparticles in the composites changed the kinetics of the drug release, as they constitute physical barriers to the drug diffusion, contributing to a sustained drug release process. Furthermore, in the presence of a magnetic field, the magnetic nanoparticles were able to perform a magnetic constriction of the material, making the drug release faster than in the absence of the magnetic field, which may be useful to perform a fine tuning of the system, allowing the easier adjustment of the speed and amount of released drug, useful to improve medical treatments and even the welfare of the patients. POLYM. ENG. SCI., 53:2308–2317, 2013. © 2013 Society of Plastics Engineers