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Frequently Asked Questions about Thimerosal

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 children’s 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.

Frequently Asked Questions


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 illness—or even death.


Why do some people worry about thimerosal in vaccines?

You may have heard that thimerosal has mercury in it. Not all types of mercury are the same. Some types of mercury, like mercury in some kinds of fish, stay in the human body and can make people sick. Thimerosal is a different kind of mercury. It doesn’t stay in the body, and is 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 thimerosal in vaccines for many years. They haven’t found any evidence that thimerosal causes harm.


Is thimerosal still used in vaccines for children?

No. Thimerosal hasn’t been used in vaccines for children since 2001.

However, thimerosal is still used in some flu vaccines. Yearly flu vaccines are recommended for all 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 in vaccines?

Most people don’t have any side effects from thimerosal, but some people will have mild side effects like redness and swelling at the place where the shot was given, which will 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?

Ask your doctor or pharmacist.

Ask to see the vaccine’s list of ingredients. All vaccine packages come with information (called an insert) that lists the ingredients.

Check out this complete list of vaccines to see which ones contain thimerosal.

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Please see Thimerosal Publications for a list of scientific articles on thimerosal.

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