I read it has anti-hydrotic properties and can be used for preventing and treating excessive sweating, but I cannot find if any research has been conducted that confirms the efficacy.
Salvia is a herbal treatment with a long tradition for this indication.
The HMPC has classified sage leaves as a traditional herbal medicinal product.
ESCOP: internally for increased secretion of sweat; externally for inflammation of the mucous membrane of the mouth and throat, gum inflammation.
Commission E: internally for dyspeptic complaints and increased secretion of sweat; externally for inflammations of the mouth and throat mucosa.
That means three different bodies, the Herbal Medicinal Product Committee (HMPC), the European Scientific Cooperative on Phytotherapy (ESCOP), and the older and defunct Kommission E, came to the conclusion that indeed sage does "help" with excessive sweating. What they didn't address was how much sage helped.
The scientific report from Kommission E was published in "Salviae folium / Salbeiblätter", Bundesanzeiger No 90, 15.05.1985. But it seems to be not online anywhere.
Going by personal experience with sage leaf tea I'd say: it helps quite a lot in a hot summer in people probably not diagnosed or suffering the condition in question. But a German medical newspaper cites:
Professor Karin Kraft: In hyperhidrosis, preparations from Dalmatian sage (Salvia officinalis L.) are mainly used as a herbal option. The antihidrotic mode of action is not known exactly, the tannins contained may be involved.
In the case of focal hyperhidrosis, patting with Salbeitinktur (50 vol.% ethanol) is recommended in folk medicine.
For generalised sweating, tea infusions with sage leaves are helpful: pour 150 ml boiling water over a teaspoon of cut leaves (2g) or a filter bag, allow to infuse for 10 minutes, strain, drink a cup of cooled tea three times a day for two to four weeks.
In addition, there is a finished drug with aqueous sage leaf dry extract. For day sweat, for example during the menopause, 1 - 2 dragées are recommended 3 times daily after meals, for nervous night sweat 2 - 4 dragées are recommended before going to bed.
The European Medicines Agency (EMA) assigns sage leaf preparations, among other things, the traditional use to relieve excessive sweating; persons up to 18 years of age, pregnant women and nursing mothers should be excluded due to lack of examination data.
The available clinical studies are always open, most of which were already carried out in the 1930s in patients with TB, for example, or in healthy volunteers.
They all support the long-standing assumption that aqueous sage leaf extracts inhibit perspiration in hyperhidrosis.
In an open, uncontrolled study, 40 patients with idiopathic hyperhidrosis received an aqueous dry extract (440mg, corresponding to 2.6g sage leaves) and a further 40 patients received sage tea (4.5g leaves per day).
Sweat secretion dropped to less than 50 percent in both groups.
A therapy trial with sage leaf extract is therefore justified in patients with idiopathic focal or generalized hyperhidrosis.
But that professor is obviously a fan of herbals and mostly quotes very old studies. Newer studies with updated protocols and standards are like: First time proof of sage's tolerability and efficacy in menopausal women with hot flushes, 2011.
When looking at Palmar hyperhidrosis: clinical, pathophysiological, diagnostic and therapeutic aspects we find that Palmoplantar Hyperhidrosis: A Therapeutic Challenge. As the International Hyperhidrosis Society remarks on Sweaty Hands:
While most people find that antiperspirants, iontophoresis, Botox injections, or a custom combination of these are enough to manage excessive hand sweating, there are those who seek a more definitive course of action.
Seeking to add Salvia into a treatment course seems to offer a weak but viable opportunity. Relying on sage alone may be not enough in most of the more severe cases.