In this recent medical study, castor oil was found to exert cytotoxic effects on conjunctival cells. So it is the castor oil. In this study, other oils were used that did not cause cytotoxicity.
http://www.dryeyezone.com/talk/showthread.php?t=4458
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http://www.dryeyezone.com/talk/showthread.php?t=4458
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