A mustard plaster is a poultice of mustard seed powder spread inside a protective dressing and applied to the body to stimulate healing. It can be used to warm muscle tissues and for chronic aches and pains. It was once part of conventional medical treatment, and available in prepared versions in pharmacies. It fell from favor in the 20th century and is now only used as a home remedy.
How it works
An enzymatic reaction in the wet mustard powder produces a chemical called allyl isothiocyanate, which is absorbed through the skin as a transdermal drug. It provides warmth and functions as a counterirritant, meaning that it stimulates nerve endings in the skin and thereby distracts the body from deeper-seated pain.
Mustard plasters were used for aches and pains, including rheumatism, arthritis, and sore muscles. It was also used for chest congestion.
If left in place for too long, it can produce significant first-degree burns to the skin.