Mexoryl SX is a synthetic sunscreen developed and patented by L’Oreal and used in their sunscreen products sold outside the United States since the early 90’s.
In 2006, the FDA approved Mexoryl SX in the United States, but only in ONE sunscreen: La Roche-Posay’s Anthelios SX SPF 15. Mexoryl SX is listed as ecamsule on the label.
Ken Klein, former president of Cosmetech Labs and teacher of sunscreen formulation classes for the Society of Cosmetic Chemists, maintains that although Mexoryl SX doesn’t break down after hours of sun exposure at the same rate as avobenzone, it does still degrade, losing 40% of its protective properties. (It’s important to note that all sunscreen agents break down at some point, which is why we’re always advised on the label to reapply it!)
Studies have shown that after controlled doses of UV exposure, avobenzone breaks down at a rate of 65%. However, avobenzone can be, and often is, made more stable by combining it with other active ingredients, specifically octocrylene. Outside the United States, Tinosorb (another sunscreen active) is often used to enhance the stability of avobenzone.
Gaspar L, Campos M. Evaluation of the photostability of different UV filter combinations in a sunscreen. Int. J. Pharm. 2006;307(2):123-328.
Chatelain E, Gabard B. Photostabilization of butyl methoxydibenzoylmethane (Avobenzone) and ethylhexyl methoxycinnamate by bis-ethylhexyloxyphenol methoxyphenyl triazine (Tinosorb S); a new UV broadband filter. Photochem Photobiol. 2001;72(3):401-6.