Thimerosal (also called thiomersal or merthiolate) is a compound that contains a type of mercury called ethylmercury. Vaccine makers have used it since the 1930s to stop bacteria and fungi from tainting vaccines.
On the FDA’s recommendation, manufacturers in 2001 removed or greatly reduced thimerosal in all routinely suggested childhood vaccines. This change was made as a precaution, but there’s no strong scientific evidence to show that thimerosal causes harm when used in vaccines.
What Is Thimerosal Used For?
When doctors, nurses, and other health care professionals use a needle to pierce a vial containing a vaccine, bacteria and fungi can get inside. These germs can make you sick or even be fatal. Vaccine makers add thimerosal to stop germs from growing in vials of vaccine that have more than one dose (multidose).
While most vaccines in the U.S. no longer contain thimerosal, some products may still contain it, including:
- Multidose flu vaccines
- Immune globulin, a drug made from human blood that boosts your immune system to treat infection or disease
- Drugs used to offset the effects of venom from animal bites or stings
- Substances used in allergy testing called skin test antigens
- Some medications in the form of nasal sprays, eye solutions, and ear drops
- Antiseptic sprays
Are there other side effects of thimerosal?
It’s unlikely that you’ll have side effects from thimerosal, but you may notice slight redness or swelling where you had a vaccination. This can last for a day or two. Some people also have an allergic reaction to thimerosal, but it’s not common.
Which Vaccines Contain Thimerosal?
While vaccines for kids under 6 don’t have thimerosal, some adult vaccines do. Multidose flu vaccines can have thimerosal. One single-dose tetanus and diphtheria vaccine made by Mass Biologics uses thimerosal during manufacturing. Thimerosal-free versions of these vaccines are usually available.
If you want to know for sure whether the product you’re using contains the compound, ask your doctor or pharmacist, or check the package insert that lists its ingredients. Look for products with these names:
- Thimerosal or mercurothiolate
- Merthiolate
- Sodium ethylmercurithiosalicylate
- Sodium2-(ethylmercurithio) benzoate
- Mercury, ethyl(2-mercaptobenzoato-s)-, sodium salt
- Mercury, ((o-carboxyphenyl)thio)ethyl-, sodium salt
Is Thimerosal Dangerous?
Scientific studies dating back many years show that the use of thimerosal in vaccines is safe.
Can thimerosal cause mercury poisoning?
There are two types of mercury you can be exposed to: methylmercury and ethylmercury. Methylmercury is the kind you find in certain types of fish. It can be toxic in high amounts.
But ethylmercury, the type in thimerosal, breaks down and passes through your body much faster. This means it’s much less likely to build up and cause health problems, such as mercury poisoning.
Can thimerosal cause autism?
There’s no scientific evidence showing a connection between thimerosal in vaccines and the brain development disorder autism. In fact, the rate of autism rose even after the U.S. removed the compound from childhood vaccines.
Can thimerosal cause cancer?
Researchers who’ve studied the effects of thimerosal haven’t found any link between the compound and health problems, such as cancer.
Does thimerosal cause birth defects?
As with autism and cancer, there’s no scientific proof that thimerosal causes birth defects. Having an open dialogue with your doctor can help ease vaccine concerns, says Kara Wada, MD, an immunologist and founder of The Immune Confident Institute in Columbus, Ohio.
“The biggest thing is understanding where [patients’] fears are coming from, what experiences they’ve had, what things they’ve heard, and to listen,” she says. “And then to say, ‘I hear your lived experience and those fears. I also have the privilege of understanding the science — Is it OK if I share that with you?’”
What Is a Thimerosal Allergy?
It’s uncommon, but you could have an allergy to thimerosal. It’s a contact allergy (also called a hypersensitivity reaction), meaning your skin reacts to the substance. You may notice itching, swelling, redness, and blisters filled with fluid. These symptoms could take many days to appear.
A skin patch test, in which you’re exposed to small amounts of possible allergens, can show if you’re allergic to thimerosal. Talk to your doctor about this test.
Why Is Thimerosal Controversial?
Starting in the late 1990s, people began voicing concerns about the safety of mercury in vaccines and their potential harm to children.
“There’s been an undercurrent of concern because thimerosal contains mercury as a part of its chemistry,” Wada says. “In rare situations where people have had extremely high exposure to thimerosal, it can cause mercury poisoning. So when there were alternatives available to use as preservatives in vaccines, it was switched out.”
At the time, the American Academy of Pediatrics and other medical groups suggested that thimerosal be removed from all routine childhood vaccines, saying that the risks of unvaccinated kids outweighed any benefits of thimerosal.
Now, U.S. Health Secretary of Health and Human Services Robert F. Kennedy Jr. has approved an advisory panel’s recommendation that only thimerosal-free single-dose flu shots be given in the U.S. The Advisory Committee on Immunization Practices (ACIP), which advises the CDC, suggested the change, raising fears about the ingredient despite its decades-long safety record.
Kennedy, who selected members of the ACIP, has falsely claimed that there’s a link between vaccines and autism.
Critics of the ACIP recommendation argue that thimerosal in vaccines is a nonissue and banning it chips away at the public’s confidence in vaccines. Currently, 94% of flu vaccines contain little or no thimerosal.
While multidose flu vaccines are still approved for use by the FDA, Kennedy’s action may mean that insurers will no longer cover them.
Are There Alternatives to Thimerosal?
While some adult vaccines and other products contain thimerosal, you can probably find options free of this ingredient. Ask your doctor or pharmacist, or check the ingredient list, for more information.
Takeaways
Thimerosal is a mercury-based compound used in vaccines to prevent tainting by bacteria and fungi, especially in multidose vials. It’s no longer included in routine childhood vaccines in the U.S. but is still present in some flu vaccines and other products. Studies show that thimerosal is safe and not linked to autism, cancer, or birth defects, though rare allergic reactions can happen. If you have a thimerosal allergy, talk to your doctor or pharmacist about other options.
FAQs About Thimerosal
Below are some commonly asked questions about thimerosal.
Do eye drops contain thimerosal?
The use of thimerosal in eye drops has tapered off, but some still contain the ingredient.
What should I avoid with a thimerosal allergy?
If you have a thimerosal allergy, avoid products that contain the preservative, such as certain vaccines, antivenom, and eye, ear, and nose medication. Check the product insert or ingredient list, or ask your doctor or pharmacist if the product you’re using is thimerosal-free.
Why do they not use thimerosal in certain vaccines?
Misleading claims surfaced that thimerosal in vaccines is harmful to children. This led health experts to suggest removing the ingredient to prevent parents from avoiding vaccines for their kids.
