About this Chemical

  • What is thimerosal? Thimerosal is an organic mercury-based preservative used in some vaccines, including some influenza vaccines. Thimerosal contains ethyl mercury. This is a different chemical compound than mercury found in the environment (methylmercury).
  • Thimerosal has been used as a preservative since the 1930s to prevent bacterial growth in vials containing multiple doses of vaccine and other medical products. Thimerosal is also used as a preservative in other medical products such as some throat and nose sprays.
  • Is there thimerosal in flu vaccines?  Yes. The majority of influenza vaccines distributed in the United States contain thimerosal as a preservative. However, an increasing amount is thimerosal-free. While the multiple dose vials still have thimerosal to prevent bacterial growth, the single-dose preparations do not. The amount of influenza vaccine that is thimerosal-free is growing every year..