poligeenan, also known as degraded carrageenan, was shown in some studies to be carcinogenic. Some high profile foods had carrageenan and there was some bad publicity due to it.

I am not a doctor, but I do believe that carrageenan has been classified as perfectly safe by the FDA.

EDIT: http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRS...