Antiperspirants are products for external use containing substances that reduce perspiration.
There are several compounds which are able to clog the outlet ducts of sweat glands, thus inhibiting sweat secretion. Among these, aluminum salts of chlorine ions are dominating in products against excessive perspiration.
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Aluminium chloride (AlCl3)
5-hydroxy-aluminum chloride
(AlCl3OH5)
Di-aluminum-hexachloride
(Al2Cl6)
The various aluminum compounds (aluminum chloride, aluminum zirconium compounds, etc.) seem to be equivalent in effect.
Astringent substances (tannins), like formaldehyde, glutaraldehyde, tannic acid or trichloroacetic acid, are rarely used today. 
Formaldehyde often caused sensitation of the skin, glutaraldehyde and tannic acid stain the skin.
An exception is methenamine (hexamethylene-tetramine), releasing ammonia and formaldehyde in acidic environment, which is better tolerated than pure formaldehyde. In addition to the use in recurrent urinary tract infections (in tablet form) the substance, used externally as an ointment (Antihydral®), is also effective for hyperhidrosis.
Applied directly to the skin, the active agent causes narrowing or closure of the outlet ducts of the sweat glands, either by swelling of the cells lining the duct, or through formation of a gel-like clot. The effect usually lasts for several hours, until the swelling subsides, the increasing pressure of the sweat overcomes the resistance of the clog and the outlet is rinsed free.
The effect depends mainly on the concentration of aluminum. Preparations with a concentration of less than 10% of aluminum salt in aqueous or alcoholic solution are considered relatively weak. As a rule, the following concentrations of aluminum salts are recommended:
| armpits: | 10-20% | ||
| palms: | 15-25% | ||
| feet: | up to 30% |
The antibacterial effect inhibiting the development of odor is mainly achieved by antiseptic substances added to antiperspirants, in particular alcohol (ethanol).
Some products also contain additives to mitigate the irritant effect of aluminum chloride on sensitive skin.
Antiperspirants for external use are available in different preparations (cream, spray, solution/lotion, roll-on). The product should be applied at night and washed off thoroughly the following morning. Sweating ceases in fact at night, allowing the active substance to penetrate into the pores without being rinsed off by sweating.
In the beginning, a daily application is usually necessary. After 1-2 weeks, it is advisable to increase the intervals between applications in order to ease the irritative effect on the skin.
The most common adverse side effect is irritation of the skin, causing reddening of the skin, itching and/or a burning sensation. If the treatment is pursued without interruption, chronic dermatitis (eczema) may develop. Lighter irritation may subside by extending the intervals between applications, possibly supported by applying a mild cortisone cream. More severe skin reactions due to high sensitivity to aluminum salts require interruption of the treatment.
In the past, aluminum-containing antiperspirants have been suspected for having a role in the pathogenesis of Alzheimer's disease or breast cancer. It has been proven, however, that the amount of aluminum ions absorbed through the skin is extremely low, even in presence of skin lesions, and indeed orders of magnitude lower than the amount absorbed through the digestive tract (e.g. with drinking water). Even long-time use of antiperspirant can therefore be regarded as safe.