Is it safe to use magnesium hydroxide as an anti-perspirant?

Question

Is it safe in the long-term to use magnesium hydroxide (commonly known as "milk of magnesia") as an anti-perspirant?

I can find some informal descriptions using magnesium hydroxide as a deodorant, but nothing scientific.

Answer 1

While official government recommendations seem to indicate a very low risk:

Toxicological Risks of Selected Flame-Retardant Chemicals. (National Research Council (US) Subcommittee on Flame-Retardant Chemicals. Washington (DC): National Academies Press (US); 2000.)

TOXICOKINETICS

Absorption: Dermal
No information was found on the dermal absorption of Mg(OH)2 in humans or experimental animals.

HAZARD IDENTIFICATION

The toxicology database on Mg(OH)2 is sparse. Therefore, the subcommittee decided that this review should also include toxicity data on magnesium salts that are expected to dissociate in stomach acid to Mg2+ cations. These include MgCl2, magnesium lactate, and magnesium citrate.

Dermal Exposure: No studies were found on the toxic effects of Mg(OH)2 following dermal exposure.

this is for the use case flame retardant. Consulting a cosmetics database it says:

MAGNESIUM HYDROXIDE
Health Concerns of the Ingredient:

• Overall Hazard low to moderate
• moderate use restrictions
• enhances skin absorption

Concern
Violation of industry recommendations

• Restricted in cosmetics; use, concentration, or manufacturing restrictions
• Avoid or minimize skin exposure: recommendation from industry safety panel (Cosmetic Ingredient Review, CIR) (only for products for use on skin)

Determined safe for use in cosmetics, subject to concentration or use limitations - Safe for use in cosmetics with some qualifications

That seems to indicate that if that ingredient is found in a commercial formulation the overall risk this substance alone represents seems to be very low. Potential interplay with other ingredients needs to accounted for. Using this in homemade ointments needs some precautions.
Using this in the long term and at higher concentrations is not recommended. While beneficial in the sense that as an absorbent it reduces excess moisture to a degree, this substance is also alkaline (pH 10.5), human skin is slightly acidic, and it needs to be. MgOH raises the pH. That is not good.