i·so·tre·ti·noin (??s?-tr?t?n-oin?)n. A synthetic isomer of retinoic acid that inhibits sebaceous gland secretion and is used in the treatment of severe nodular acne. It can cause birth defects if taken during pregnancy.[iso- + tretinoin, retinoic acid (t(rans)- + retino(ic acid) + -in).]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.isotretinoin (?a?s??tr??t?n???n) n (Pharmacology) a drug related to vitamin A, used to treat severe acne that has failed to respond to other treatmentCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014