Calcium propionate has been studied extensively for toxicity and as a potential cancer-causing agent in the laboratory, and all the findings were negatives. Only young and vitamin B12 deficient rats that were fed enormous amounts of calcium propionate (many times more than is possible from a normal diet) in one study showed a decrease in growth rates. Rats fed large amounts of calcium propionate and similar chemicals over longer periods of time have not shown any negatives effects. The accumulation of research evidence shows that calcium propionate is non-toxic and safe to use in the typical amounts currently used by food manufacturers.
There are anecdotal claims that some people who are sensitive to calcium propionate may suffer from migraine headaches triggered by exposure to foods that contain it. However, there does not appear to be any medical research that backs that claim. Calcium propionate does not build up in any body tissues; after you eat food with calcium propionate, the body splits the calcium apart from the propionic acid, which is easily absorbed and metabolized like all other fatty acids.
Calcium Propionate. United States Food and Drug Administration. Accessed July 27, 2009. http://www.accessdata.fda.gov/scripts/fcn/fcnDetailNavigation.cfm?rpt=scogsListing&id=62
CHEMINFO: Calcium Propionate. Canadian Centre for Occupational Health and Safety. Accessed July 27, 2009. http://www.intox.org/databank/documents/chemical/calcprop/cie654.htm