Ethylmercury solution is primarily used as a preservative in certain medical products, particularly in vaccines, to prevent contamination by bacteria and fungi. Its antimicrobial properties help maintain the sterility of multi-dose vials, ensuring the safety and efficacy of the vaccines over time. It is also employed in some diagnostic reagents and laboratory solutions where microbial growth must be controlled. However, its use is carefully regulated due to potential toxicity concerns, and alternatives are often sought in many applications to minimize health risks.