Magnesium Hydroxide

Magnesium hydroxide is an inorganic base used mainly as a pH adjuster/buffering or absorbent; when present in leave-on products (often low percentages, but occasionally higher in deodorants), it can raise local pH and disrupt an already-compromised barrier. Clinical experience and patch testing show it is not a common allergen, but alkaline exposure can cause stinging, dryness, and irritant dermatitis in eczema-prone or freshly-shaved/occluded skin, so I rate it as mild rather than “very gentle.” Safety Notes: In consumer skincare, magnesium hydroxide appears at very low levels (~0.05–0.5%) as an alkalinity buffer/odor-adsorbing particulate in leave-on lotions, creams, and some acne/soothing products where pH control and sensory are limiting. Higher levels are found in OTC deodorant creams/sticks and “milk of magnesia”-inspired leave-on products, commonly ~5–15% and reaching ~20% in high-solids suspensions designed for strong odor control/mattifying effects. Use above a few percent is typically restricted to high-alkaline, particulate-rich systems and is more common in leave-on deodorant-type formats than in facial skincare; rinse-off cleansers generally sit on the low end due to viscosity, stability, and pH constraints.