Neostigmine Methylsulfate is a chemical compound documented solely within pharmaceutical and regulatory contexts. It functions as a prescription drug ingredient, utilized medically to reverse neuromuscular blocking agents post-surgery and to manage conditions such as myasthenia gravis and urinary retention.

The compound has also been identified as an unauthorized additive in certain non-prescription eye care products, specifically the Japanese brands *Sante FX Neo* and *Mytear Fresh40*. In these eye drops, it is included to induce pupil constriction, aiming to alleviate symptoms like eyestrain and blurred vision.

Health authorities, including Health Canada, have issued warnings regarding its use in non-prescription eye solutions. These advisories highlight potential risks such as cataracts, retinal detachment, glaucoma attacks, and cardiac or respiratory complications, stemming from unintended systemic absorption of the compound.