Magnetically responsive, controlled-release drug formulations are needed for targeted drug delivery, especially for toxic drugs. A supercritical antisolvent (SAS) technique is used to produce magnetically responsive poly(methyl methacrylate), 50/50 DL-poly(lactide-co-glycolide), and Eudragit RS polymer particles via coprecipitation of the polymer with a suspension of magnetite particles in mineral oil and a fatty acid surfactant, using dichloromethane as the solvent. The SAS technique is again used to precipitate drug-loaded magnetically responsive polymer particles. A modification of the SAS process, the SAS-EM technique developed at Auburn University, is used to produce magnetite-encapsulated polymer nanoparticles of controllable sizes. In SAS-EM, atomization and increased mixing within the supercritical phase is brought about using a surface vibrating at an ultrasonic frequency. Size, morphology, and drug-release kinetics of the obtained particles are studied.