Thimerosal: A mercury-containing preservative used in some vaccines and other products since the 1930's. No harmful effects were reported from thimerosal at doses used in vaccines, except for minor local reactions like redness and swelling at the injection site. There was specifically no evidence that thimerosal increases the risk of developing autism or any other behavior disorder. However, in 1999 it was agreed that thimerosal should be reduced or eliminated in vaccines as a precautionary measure. Today, all routinely recommended pediatric vaccines in the US contain no thimerosal or only trace amounts.