 Pharmacia & Upjohn ( now part of Pfizer ) started its own oxazolidinone research program in the 1990s . Studies of the compounds ' structure – activity relationships led to the development of several subclasses of oxazolidinone derivatives , with varying safety profiles and antimicrobial activity . Two compounds were considered drug candidates : eperezolid ( codenamed PNU @-@ 100592 ) and linezolid ( PNU @-@ 100766 ) . In the preclinical stages of development , they were similar in safety and antibacterial activity , so they were taken to Phase I clinical trials to identify any difference in pharmacokinetics . Linezolid was found to have a pharmacokinetic advantage — requiring only twice @-@ daily dosage , while eperezolid needed to be given three times a day to achieve similar exposure — and therefore proceeded to further trials . The U.S. Food and Drug Administration ( FDA ) approved linezolid on April 18 , 2000 . Approval followed in Brazil ( June 2000 ) , the United Kingdom ( January 2001 ) , Japan and Canada ( April 2001 ) , Europe ( throughout 2001 ) , and other countries in Latin America and Asia . 
