Thalidomide, a drug used during the 1950s as a sleeping pill and to relieve morning sickness in pregnant women, was a mixture of two forms: one beneficial, while the other caused severe birth defects. Chemists handing out the drug did not know this.
Links:
[1] http://admin.indiaenvironmentportal.org.in/feature-article/parting-twins
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/down-earth
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/united-states-america-us
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/drugs