Thimerosal is found in a small number of products today, primarily certain multi-dose flu vaccines, some antivenoms and immune globulins, and a few topical antiseptics. Its use has dropped dramatically over the past two decades, and many product categories that once relied on it, like contact lens solutions and eye drops, have phased it out entirely.
What Thimerosal Is and Why It’s Used
Thimerosal is a preservative that’s roughly 50% mercury by weight. It prevents bacterial and fungal contamination in products that come in multi-dose containers, where a needle or dropper enters the same vial multiple times. The mercury in thimerosal is ethylmercury, which the body processes and eliminates much faster than the methylmercury found in fish. In blood, ethylmercury has a half-life of about 7 days compared to 19 days for methylmercury, and it reaches brain tissue at concentrations three to four times lower.
Multi-Dose Flu Vaccines
The most common products containing thimerosal in the U.S. today are multi-dose influenza vaccines. These vials hold enough vaccine for multiple patients, which makes large-scale flu campaigns more practical and affordable. The specific products that contain thimerosal as a preservative at a concentration of 0.01% include:
- Afluria (Seqirus): 24.5 micrograms of mercury per standard 0.5 mL dose
- Flucelvax multi-dose (Seqirus): 25 micrograms of mercury per 0.5 mL dose
- Fluzone multi-dose (Sanofi Pasteur): 25 micrograms of mercury per 0.5 mL dose
If you prefer to avoid thimerosal, single-dose flu shots are widely available and don’t contain the preservative. You can ask your pharmacist or doctor’s office which formulation they carry.
Childhood Vaccines
Thimerosal was removed from or reduced in all routinely recommended childhood vaccines (except flu) by 2001, and the last thimerosal-preserved children’s vaccines expired in January 2003. Today, all vaccines routinely recommended for children 6 and younger in the U.S. are available in thimerosal-free formulations. The same is true for adolescent and adult vaccines.
One small exception: a single-dose tetanus and diphtheria vaccine made by Mass Biologics uses thimerosal during manufacturing but not as a preservative. Only a trace amount remains in the final product.
Antivenoms and Immune Globulins
Several biological products have historically used thimerosal as a preservative. These include antivenoms for pit viper, black widow spider, and coral snake bites, as well as certain immune globulin preparations like rabies immune globulin. Some of these products still list thimerosal on their labels. Because these are treatments for emergencies or specific exposures, they’re not products most people encounter routinely.
Eye Drops and Contact Lens Solutions
Thimerosal was once a standard preservative in ophthalmic products. Brands like Soac-Lens, Preflex, Hydrocare, and several others contained it at concentrations between 0.001% and 0.004%. None of these products are still commercially available. The shift happened because thimerosal caused allergic reactions and eye irritation in a meaningful percentage of contact lens wearers. Modern multi-purpose contact lens solutions use different preservatives entirely.
A few prescription eye drop formulations may still contain trace amounts of thimerosal, but this has become uncommon. If you have a known sensitivity, checking the ingredient list on any ophthalmic product is straightforward since preservatives are listed on the label.
Topical Antiseptics and Cosmetics
Thimerosal has been classified as an active ingredient in over-the-counter first aid antiseptic products. It also appears as a preservative in some cosmetics and topical pharmaceutical preparations. However, its use in these categories has been declining steadily. Most modern antiseptic creams and first aid products use alternative preservatives. If a product does contain thimerosal, it will appear on the ingredient label, sometimes listed by its chemical name sodium ethylmercurithiosalicylate.
Thimerosal Sensitivity Is Declining
As thimerosal has disappeared from consumer products, allergic sensitivity to it has dropped in parallel. Patch testing data from a large multi-center study in Italy found that positive reactions fell from 8.13% of tested patients in 1997 to just 0.95% in 2023. In North America, sensitization rates climbed as high as 10% in 2002, driven largely by years of exposure through contact lens solutions and vaccines. The sharp decline since then reflects how few products still contain the preservative.
People who are sensitive to thimerosal typically develop contact dermatitis, a localized skin reaction with redness, itching, or swelling at the site of exposure. This is different from a systemic allergic reaction and generally isn’t dangerous, but it can be uncomfortable enough to warrant switching to thimerosal-free alternatives.
How to Check Your Products
For vaccines, the FDA maintains a table listing every U.S.-licensed vaccine along with its thimerosal status, concentration, and mercury content per dose. Your pharmacist can tell you whether a specific flu shot is from a multi-dose vial (likely contains thimerosal) or a single-dose prefilled syringe (thimerosal-free). For over-the-counter products, thimerosal will appear in the active or inactive ingredients section of the label. If a product is labeled “preservative-free,” it won’t contain thimerosal.