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Background: Kshara (caustic alkali prepared from plant ash) is described in the Sushruta Samhita as superior to sharp instruments for certain procedures. It is used internally (paniya kshara) and externally (pratisaraniya kshara), and in the form of Kshara sutra, a medicated thread used in the treatment of fistula-in-ano and other anorectal conditions. Objective: To review the preparation and physicochemical characteristics of kshara, the standardisation of Kshara sutra, and the clinical evidence for its use. Methods: A critical narrative review of classical descriptions, pharmaceutical and analytical studies and clinical trials. Results: Classical methods dissolve plant ash in four to eight volumes of water, settle, filter repeatedly (21 times in the Sushruta Samhita) and evaporate, with additives giving mild, moderate and strong grades. Apamarga pratisaraniya kshara had pH 13.88, potassium 9.19% and sodium 0.63%, indicating a strongly caustic potassium-rich alkali. Kshara sutra is made by applying 21 coatings to surgical linen thread: 11 of Snuhi latex, 7 of latex with Apamarga kshara and 3 of latex with turmeric. A multicentric randomised trial by the Indian Council of Medical Research, a randomised trial against fistulotomy and later comparative studies support its effectiveness for fistula-in-ano, with fewer complications than fistulotomy in some comparisons, although most studies are small and methods of preparation vary. Conclusion: Kshara sutra is among the best-evidenced Ayurvedic procedures. Standardisation of kshara alkalinity, coating load and thread properties, and larger trials against modern sphincter-sparing techniques, are needed.