Sodium methylparaben is a preservative used in cosmetics, food, and pharmaceuticals. It's the sodium salt form of methylparaben, which is also known as methyl hydroxybenzoate or benzoic acid, 4-hydroxy-, methyl ester. It's a white, hygroscopic powder with the chemical formula C8H7NaO3.

Sodium methylparaben has many benefits in cosmetics, including: 

Preservation

It prevents microbial growth and germs like mold and bacteria, and preserves ingredients. 

Shelf life

It works with other parabens to make cosmetics safer and improve their shelf life. 

Non-irritating

It's considered safe to use and won't irritate the skin when applied topically. 

The FDA generally considers methylparaben safe, and acute toxicity studies in animals indicate it's non-toxic when ingested or injected. However, some people may be sensitive to it, and you can ask your doctor about a patch test if you think you might have a reaction.