What is Thimerosal?

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  • Written By: Amy Hunter
  • Edited By: Andrew Jones
  • Last Modified Date: 02 November 2019
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Thimerosal is the name of a compound that is made of approximately 50 percent mercury. It has antiseptic and antifungal properties, and is an effective preservative in vaccines. It is used in a wide range of medical products in addition to vaccines, including immunoglobulin preparations, antivenom, ophthalmic solutions, and tattoo ink.

At one time, thimerosal, under the trade name Merthiolate, was widely used in over the counter solutions and ointments for cuts and other skin conditions. It was also in contact lens solution, nasal sprays, and eye drops. It is not as prevalent today as concern over its safety has grown.

Thimerosal is a preservative used in vaccines that come packed in multiple use bottles. Using the same bottle for multiple injections over a period of several days increases the likelihood of contamination. Bacteria can grow in the vaccine solution, and, when the tainted vaccine is injected under the skin, death can result within 24 hours. Thimerosal acts as a preservative, preventing bacteria from multiplying in the solution.

In single use vaccines, a preservative is not necessary. Since the entire solution is used at one time, there is no opportunity for the solution to become contaminated. Single use vaccines are more expensive, however.


Some parents and doctors are concerned that thimerosal is responsible for autism in young children. There is no proof that this is the case. Researchers who do not feel that it is responsible for autism cite the hypothesis that the age of the onset of typical autism symptoms just happens to coincide with the time when children receive a variety of vaccines. The World Health Organization has stated that there is no medical evidence that this preservative has any affect on the development of autism.

Despite the reassurances from various health groups, vaccine manufacturers realized that people were not happy with the idea of vaccinating their children with products containing such high levels of mercury. The use of thimerosal is being phased out in childhood vaccines. Eventually, all vaccines for children will be single use vaccines.

Even without thimerosal in childhood vaccines, there is still a chance that your child could receive an injection containing the preservative. Vaccines not normally recommended for children will still contain it. Many treatments for venomous bites contain it as well. The antivenom for coral snakes, pit vipers, and black widow spiders all contain thimerosal.


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