Merthiolate is a brand name for thiomersal, a mercury-containing medicinal constituent that was used as a preservative and pathogen-inhibitor in numerous types of products, including vaccines, according to MedlinePlus. It was also used in some nasal and eye drop formulations.
The United States Food and Drug Administration banned the use of Merthiolate in over-the-counter products in the late 1990s due to its potential to initiate a wide variety of side effects. These include memory problems, vomiting, skin numbness, decreased urine output, mouth sores and swelling of the throat. Merthiolate poisoning is difficult for doctors to treat, and large-scale exposure to the substance can be fatal, notes MedlinePlus.