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Frequently Asked Questions About Thimerosal (Ethylmercury)

Some parents have questions about the safety of ingredients – like thimerosal (“THY-mayr-uh-sal”) - in children’s shots (vaccines).

We want you to know that thimerosal is no longer used in childrens’ shots, except some types of flu shots. You can ask for a flu shot without thimerosal.

Check out these answers to common questions about thimerosal.

What is thimerosal?

Thimerosal is a vaccine additive, added to some vaccines to prevent germs (like bacteria and fungi) from growing in them.

If germs grow in vaccines, they can cause infections and illness.

Why do some people worry about thimerosal in vaccines?

As you may have heard, thimerosal has mercury in it. Thimerosal contains "ethylmercury". Not all types of mercury are the same. Some types of mercury, like elemental mercury and methymercury, which are found in the environment and in some kinds of fish, stay in the human body and at high levels can make people sick. But ethylmercury is broken down more easily in the body and elliminated. It doesn't stay in the body, so it’s unlikely to make us sick.

Is thimerosal safe?

Yes. Thimerosal has been used safely in vaccines for a long time (since the 1930s).

Scientists have been studying the use of vaccines that have thimerosal in them for many years. They haven’t found any evidence that thimerosal causes harm.

Is thimerosal still used in vaccines for children?

There is no thimerosal used in the vaccines on the childhood immunization schedule. Read more about each of these vaccines:

However, thimerosal is still used in some flu vaccines. Yearly flu vaccines are recommended for all children, and flu vaccines containing thimerosal are considered safe and recommended for children. If you are worried about thimerosal, you can ask for a flu vaccine without it.

Does thimerosal cause autism?

No. Research does not show any link between thimerosal and autism.
Read more about vaccines and autism.

Are there side effects from thimerosal?

Most people don’t have any side effects from thimerosal, but some people will have mild reactions like redness and swelling at the place where the shot was given, which only last 1 to 2 days. It’s very unlikely you will have an allergic reaction to thimerosal.

How can I find out if thimerosal is in a vaccine?

  1. Ask your doctor or pharmacist.
  2. Ask to see the vaccine’s list of ingredients. All vaccine packages come with information (called an insert) that lists the ingredients.
  3. Check out this complete list of vaccines to see which ones contain thimerosal.

Remember: Shots help protect your kids from dangerous diseases. Find out which shots your children need.


Please see References for a list of published articles on thimerosal.

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