Excessive sweating: what hyperhidrosis actually is

Humans cool themselves by evaporation. Millions of eccrine glands, concentrated in the palms, soles, underarms and forehead, release a watery fluid onto the skin. As it evaporates it draws heat away from the body. It is an efficient system and it is the main reason people tolerate heat as well as they do.

A second, smaller group, the apocrine glands, sits in the underarm and groin and becomes active after puberty. These secrete a thicker fluid that is odourless until skin bacteria break it down. This distinction matters: body odour and excessive wetness are two different problems with two different sources, and treating one does not automatically resolve the other.

If you examined the sweat glands of someone with primary hyperhidrosis under a microscope, they would look entirely normal. There are not more of them and they are not enlarged. What differs is the instruction they receive.

Eccrine glands are driven by the sympathetic nervous system, the same branch that handles the fight-or-flight response. In hyperhidrosis that circuit is set too sensitively: it fires in response to mild warmth, mild stress, or nothing identifiable at all. The gland does exactly what it is told. It is simply being told too often.

This single fact explains most of what follows. It is why antiperspirants, which physically block the duct at the skin surface, help some people. It is why treatments that interrupt the nerve signal work where deodorants never could. And it is why no amount of washing changes anything: the problem is upstream of the skin entirely.

Primary focal hyperhidrosis is the common form. It has no underlying disease behind it, tends to run in families, usually appears in childhood or adolescence, affects specific areas symmetrically, and characteristically settles during sleep. It is a condition in its own right.

Secondary hyperhidrosis is sweating caused by something else: a hormonal shift, a thyroid or blood-sugar issue, an infection, a medication side effect. Here the sweating is a message, and the useful response is to find what is sending it rather than to silence the messenger.

Any responsible assessment separates these two before discussing treatment at all. The pattern usually tells the story, and the questions that distinguish them are covered in detail in our companion article on when sweating is a symptom rather than a condition.

The underarms are the most commonly treated area, but palms, soles, face and scalp are all recognised sites, and many people have more than one. The areas involved are usually consistent over years.

The impact is rarely dermatological. It shows up in clothing choices made every morning, in avoiding handshakes and greetings, in gripping a steering wheel or a phone, in professions where wet hands or visible marks are a daily concern. Studies of quality-of-life scores in hyperhidrosis consistently place it alongside conditions people take far more seriously.

There is no single laboratory test for primary hyperhidrosis. The diagnosis is clinical and rests mostly on the history: when it started, whether it is symmetrical, whether it stops during sleep, whether anyone else in the family has it, whether it is confined to specific areas, and what medications you take.

A severity scale is often used to record how much the sweating interferes with daily activity, which gives a shared reference point for judging whether a treatment has helped. Where injectable treatment is being planned, a simple starch-iodine skin test can map the most active zone so that treatment is directed accurately rather than spread evenly.

Where the history points to a secondary cause, the right next step is a medical work-up rather than a cosmetic appointment, and a clinic acting properly will say so.

Treatment is a ladder rather than a menu. Most people start at the bottom, and a good number never need to go further.

Topical antiperspirants come first, and technique changes the outcome more than the product does. Aluminium-salt antiperspirants work best applied to completely dry skin at night, when sweat production is at its lowest, and washed off in the morning. Applied to damp skin after a shower, the same product performs poorly and often stings, which is why so many people abandon it.

Injectable treatment with botulinum toxin is the established next step for focal sweating, particularly of the underarms, when topical measures are not enough. It works by interrupting the chemical signal between nerve and gland in the treated area only, so the effect is local and temporary. We explain the mechanism in detail in a separate article.

Iontophoresis, in which the hands or feet sit in shallow water carrying a weak electrical current, is a long-standing option for palms and soles and can be continued at home. Where focal treatment is not sufficient, a physician may consider oral medication that reduces sweating more generally, which requires a proper prescribing discussion because it reduces sweating everywhere, including where you need it. Device-based and surgical approaches exist for severe, resistant cases and are considered last, not first.

Several widely repeated ideas are worth setting aside. Washing more often does not reduce sweat production, because the problem is not on the skin. Stronger deodorant does not address wetness. "Sweating it out" through heat exposure does not train the condition away. And no dietary change has been shown to resolve primary hyperhidrosis, though heat, spice, caffeine and alcohol can each trigger an episode in someone who already has it.

The most persistent unhelpful idea is that heavy sweating reflects poor hygiene. It does not, and the belief that it does is a large part of why people wait years before asking anyone about it.