Is Magnesium Hydroxide Safe in Deodorant?

Magnesium Hydroxide has somehow become the "it" ingredient when it comes to formulating "sensitive skin" deodorants. Companies have taken out the baking soda and replaced with magnesium hydroxide in an attempt to make a deodorant with a reputation of being more gentle on skin. However, this decision is based purely on marketing and not in any scientific reality. 

One marketer of magnesium hydroxide (they sell magnesium hydroxide to the deodorant industry) says this: 

"Although it effectively neutralizes bacteria, baking soda is alkaline, contrasting with the acidity of skin. As a result, baking soda strips the natural moisture barrier, causing irritation, dryness, and discoloration. There is also potential for allergic reaction. Magnesium Hydroxide, found in soothing products such as Milk of Magnesia, balances the body’s natural pH while killing odor-causing bacteria without reducing skin moisture or causing skin damage."

This statement really falls apart when you look at it closely. 

They assert that because baking soda is alkaline, it strips the moisture barrier. The thing is, magnesium hydroxide is an alkali just like baking soda. It's used as an antacid (milk of magnesia) because, well, it neutralizes acid. It's an alkali, after all, with a HIGHER (more alkaline) pH than baking soda. So if baking soda is an irritant due to its alkalinity, magnesium hydroxide would be even more irritating. 

Magnesium hydroxide has a typical pH in solution of 9 to 10.5 (Source)(Source).

Baking Soda has a typical pH of 8.3 to 9 in solution. (Source)(Source)

Magnesium hydroxide has also been found to be a skin sensitizer. (Source) Baking soda has not. (Skin sensitization means that skin gradually develops an intolerance to an ingredient.) 

True allergic reactions to baking soda are really quite rare. If you're allergic to baking soda, you'd also be allergic to your own sweat. Both sodium and bicarbonate ions (ie dissolved baking soda) exist naturally in sweat and in our bodies. (Source) Allergic reaction is not listed as a potential side effect for drug use of baking soda. (Source) In fact, baking soda is a common treatment for allergic reactions (Source) and recommended for soothing irritated and itching skin. (Source) It's even used for relief in psoriasis treatment. (Source) In animal studies, it was also found not to be a skin irritant. (Source) 

So how did deodorant companies start using magnesium in deodorants? In about 2013 using milk of magnesia as deodorant became a viral internet trend. So many people were talking about it, companies figured they'd cash in on the trend. A few small producers started making magnesium hydroxide formulas and then the trend caught on with some of the corporate-owned brands. So now magnesium hydroxide deodorants are being marketed as "sensitive skin" options when they really aren't. 

It is possible to react to baking soda, but it's not typically due to pH. Baking soda (as well as magnesium hydroxide) is largely insoluble in water. So, if used at too high a concentration, that grainy sharp texture will increase underarm friction or create microscopic lesions on your skin that turns into a red/dark patch as it sits there and rubs. It's more about the *overall texture* of the formula and the friction in your underarm. Read more here.