Astringent describes any substance that causes living tissue to contract by precipitating surface proteins. In equine medicine, the clinically relevant astringents are tannic acid, zinc oxide, zinc sulfate, and alum. Each acts by cross-linking proteins in the superficial layers of skin or mucous membrane, reducing local blood flow and secretion and producing a drying, tightening effect on the tissue.
Zinc sulfate is the active ingredient in the hoof-hardening solutions used by farriers and owners to toughen a weak, soft hoof wall and frog. A softened frog in a wet environment is vulnerable to thrush; daily application of dilute zinc sulfate solution contracts the tissue and reduces the anaerobic pockets where Fusobacterium necrophorum proliferates. Zinc oxide appears in many topical wound preparations, where it protects raw tissue from moisture and provides mild antibacterial action.
Tannic acid is less commonly used in modern equine practice but was historically applied to burns and proud flesh to inhibit granulation tissue overgrowth. The mechanism is the same — protein precipitation tightens and desiccates the surface. Any astringent application to open wounds carries the risk of delaying healing if applied too aggressively; the correct use is to control excess secretion or tissue proliferation, not to dry a wound that needs moisture for epithelialization. A veterinarian’s guidance governs treatment of anything beyond superficial skin management.
Further reading: Astringent on Wikipedia; Astringent substances at Britannica.