Used primarily as an alkylating agent in organic synthesis, enabling the introduction of methyl groups to various nucleophiles such as amines, thiols, and enolates. It is particularly effective in the preparation of quaternary ammonium compounds and sulfonium salts due to its high reactivity. Commonly employed in pharmaceutical manufacturing for modifying active ingredients and intermediates where selective methylation is required. Its sulfonate ester structure provides good leaving group ability, making it suitable for reactions under mild conditions. Often chosen over other methylating agents when higher selectivity and reduced side reactions are needed. Caution is advised as it is toxic and potentially carcinogenic.