As nouns the difference between thalidomide and phthalidomide
is that thalidomide
is (pharmaceutical drug) oxopiperidin-3-yl)-1h-isoindole-1,3(2h)dione
— a drug that was sold during the late 1950s and 1960s as a sleeping aid, and to pregnant women as an antiemetic to combat morning sickness and other symptoms, but was withdrawn after being proven to cause severe birth defects, such as phocomelia; currently used to treat leprosy while phthalidomide
is alternative spelling for thalidomide.