Also known as: thiomersal, ethylmercury, merthiolate
Thimerosal
Sourced from FDA-approved package inserts (DailyMed). Last verified: 2026-07-07.
What It Is & Why It's in Vaccines
Thimerosal is an organic mercury-containing compound (sodium ethylmercurithiosalicylate) used as a preservative in some multi-dose vaccine vials to prevent bacterial and fungal contamination. It has been used in vaccines since the 1930s.
Preservative — prevents microbial growth in multi-dose vials. A single contaminated multi-dose vial can cause serious infections when reused across multiple patients. Thimerosal is not needed (and is not present) in single-dose vials since the vial is used only once.
Dose-in-Context
Per-Dose Quantity
≤25 mcg mercury per 0.5 mL dose (when present in multi-dose formulations). Most current single-dose pediatric vaccines contain no thimerosal.
Reference Comparison
Thimerosal contains ethylmercury, not methylmercury — these are chemically distinct compounds with different toxicokinetic profiles. Ethylmercury is eliminated from the body significantly faster than methylmercury (blood half-life ~7 days vs ~50 days), and does not accumulate in the CNS to the same degree as methylmercury. EPA/FDA reference doses for methylmercury (the form found in fish and the environment, with well-established neurodevelopmental toxicity at sufficient exposure levels) are not directly applicable to ethylmercury from thimerosal-containing vaccines.
Source: Pichichero ME, Cernichiari E, Lopreiato J, Treanor J. Mercury concentrations and metabolism in infants receiving vaccines containing thimerosal: a descriptive study. Lancet. 2002;360(9347):1737-1741. doi:10.1016/S0140-6736(02)11682-5
Route-of-Administration Note
Injected route. Pichichero et al. (2002) measured blood mercury concentrations in infants after thimerosal-containing vaccines and found levels below EPA reference values for methylmercury (which, again, is not the same compound), with rapid clearance consistent with ethylmercury's shorter half-life.
Documented Adverse Effects
Local hypersensitivity reactions (redness, swelling at injection site). Thimerosal can act as a contact sensitizer — some individuals develop allergic contact dermatitis to thimerosal with repeated topical exposure, but this does not appear to translate to systemic hypersensitivity with the trace quantities in vaccine doses.
The thimerosal-autism hypothesis has been one of the most extensively investigated questions in vaccine safety research. Multiple large-scale epidemiological studies across multiple countries (Denmark, Sweden, UK, US, Canada — involving hundreds of thousands to millions of children), systematic reviews by the Institute of Medicine/National Academy of Medicine (2004), Cochrane reviews, and independent analyses have consistently found no causal association between thimerosal-containing vaccines and autism spectrum disorder. Thimerosal was removed from most US childhood vaccines as a precautionary measure in 1999–2001; autism diagnosis rates have continued to rise in the period since thimerosal removal, which is inconsistent with a causal role.
Some hypotheses about subtle neuropsychological effects of low-level ethylmercury exposure have been raised in small studies. The evidence base is thin — most well-powered studies with adequate confounder control have not found effects. The methodological challenge is that thimerosal removal from the routine pediatric schedule makes it difficult to study contemporary populations with and without exposure.
Vaccines Containing Thimerosal
Influenza — Fluzone Quadrivalent (Sanofi Pasteur)
Always check vial type: preservative status differs between single-dose and multi-dose presentations.
ⓘ Methodology Disclaimer: Ingredient lists are sourced from official FDA-approved package inserts for the specific branded products named on each vaccine page. Formulations can change between manufacturing lots and over time — always verify against the current package insert before making a clinical decision. The presence of a substance in a vaccine does not by itself indicate harm; toxicology is dose-, route-, and context-dependent. Comparisons to reference exposure levels are provided with their sources where available, and are approximate — see the cited source for full methodology.