Also known as: formalin, methanal
Formaldehyde
Sourced from FDA-approved package inserts (DailyMed). Last verified: 2026-07-07.
What It Is & Why It's in Vaccines
Formaldehyde is a simple, naturally occurring organic compound present in the environment and produced continuously by normal human metabolism as part of one-carbon metabolism (the folate cycle). It is used in vaccine manufacturing to inactivate viruses (for inactivated viral vaccines) or detoxify bacterial toxins (for toxoid vaccines like tetanus and diphtheria).
Inactivating agent in the manufacturing process. Formaldehyde cross-links viral proteins and nucleic acids, rendering the pathogen incapable of replication while preserving the antigenic structure needed to generate an immune response. Most formaldehyde is removed during purification; trace residual amounts may remain in the final product.
Dose-in-Context
Per-Dose Quantity
≤0.1 mg (100 mcg) per dose (residual). The FDA limit for residual formaldehyde in vaccines is ≤0.02% (0.2 mg/mL).
Reference Comparison
Formaldehyde is a normal product of human metabolism. The human body produces and metabolizes approximately 50 grams of formaldehyde per day as part of normal one-carbon metabolism. An adult human has approximately 2–3 mcg of formaldehyde per mL of blood at baseline (endogenous concentration). A vaccine dose containing 100 mcg of formaldehyde — equivalent to 0.0001 g — represents approximately 0.0002% of daily endogenous formaldehyde production. The human body metabolizes this quantity in well under a minute via the formaldehyde dehydrogenase pathway.
Source: Heck Hd, Casanova M. The implausibility of leukemia induction by formaldehyde: a critical review of the biological evidence on distant-site toxicity. Regul Toxicol Pharmacol. 2004;40(2):92-106. doi:10.1016/j.yrtph.2004.05.001. Dhareshwar SS, Stella VJ. Your prodrug releases formaldehyde: should you be concerned? No! J Pharm Sci. 2008;97(10):4184-4193. doi:10.1002/jps.21319.
Route-of-Administration Note
Injected route. Most formaldehyde toxicology reference values (EPA IRIS inhalation Reference Concentration, OSHA workplace exposure limits) are for the inhalation route and are not directly applicable to injected trace quantities given the rapid metabolism and endogenous baseline. Formaldehyde is classified as a known human carcinogen by IARC and NTP based on inhalation-route epidemiological evidence (nasopharyngeal cancer in occupational cohorts with chronic high-level inhalation exposure); this classification is route-specific and exposure-level-specific and does not transfer to the injected trace quantities in vaccines without pharmacokinetic adjustment.
Documented Adverse Effects
At the trace residual levels present in vaccines, formaldehyde has not been associated with systemic adverse effects. The quantities are orders of magnitude below endogenous production.
In animal toxicology studies, formaldehyde is a known carcinogen by the inhalation route at high doses. This is not directly applicable to injected trace quantities in vaccines, but the distinction should be stated explicitly.
Formaldehyde hypersensitivity (contact dermatitis) is well-described with topical exposure; systemic hypersensitivity to injected trace formaldehyde from vaccines has not been established. The evidence base is sparse.
Vaccines Containing Formaldehyde
Hepatitis B — Recombivax HB (Merck Sharp & Dohme LLC)
Aluminum salt is AAHS (chemically distinct from aluminum hydroxide used in Engerix-B).
DTaP/Tdap — Daptacel (Sanofi Pasteur)
DTaP/Tdap — Infanrix (GlaxoSmithKline Biologicals)
Polio (IPV) — IPOL (Sanofi Pasteur)
Inactivated whole-virus; no aluminum adjuvant.
Influenza — Fluzone Quadrivalent (Sanofi Pasteur)
Always check vial type: preservative status differs between single-dose and multi-dose presentations.
Hepatitis A — Vaqta (Merck Sharp & Dohme LLC)
Hepatitis A — Havrix (GlaxoSmithKline Biologicals)
Japanese Encephalitis — Ixiaro (Valneva Austria GmbH)
ⓘ 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.