Educational information, not medical advice — ask your doctor or midwife.
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Black mustard is the seed of a tall yellow-flowered plant. Old labels call it Sinapis nigra. Botanists now file the same plant as Brassica nigra.
Its medical form was the mustard plaster. Mustard flour was mixed with water, spread on cloth, and laid on the chest or back. The skin turned hot and red within minutes.
The skin reaction is real and easy to see. Good human evidence that it treats a cough, a cold or a sore joint is lacking. Long use in a tradition is not proof of effect, and it is not proof of safety.
Black mustard works by irritating the skin. That part is not in doubt, and it is also where the danger sits.
Dry seed holds a compound called sinigrin plus an enzyme called myrosinase. Crushing the seed and adding water lets the two meet. The reaction makes allyl isothiocyanate, the sharp oil you smell.
That oil excites nerve endings in the skin. Blood vessels widen, so the patch turns red and feels warm. In laboratory work the oil switches on a nerve sensor called TRPA1, the same one that reacts to wasabi.
The old reasoning was counter-irritation. The idea was that a sting on the skin would pull pain and congestion away from the organ or joint below. That idea has never been tested properly in people.
Folk writers also said the warmth loosened chest mucus. There is no good human trial showing that mustard on the chest clears a cough or shortens a cold.
The burn is easier to explain than the benefit. If the oil stays in contact too long, it keeps destroying skin cells. Pain, blisters and deep burns can follow, and they can keep getting worse after the cloth comes off.
Black mustard reaches you in several forms. Most are ordinary food. A few are strong enough to burn skin.
Whole and ground seed sit in the spice aisle. Table mustard is the paste made from them.
Mustard flour, also sold as mustard powder, is the plaster base. It is finely ground seed with the hull taken off.
Ready-made mustard plasters and mustard papers were once sold by drug firms. They are hard to find today.
Mustard foot baths and bath powders stir a spoonful of mustard flour into warm water. They are a milder version of the same idea.
Mustard essential oil is a different product. It is close to pure allyl isothiocyanate. In concentrate it blisters skin and is dangerous to swallow.
Cold-pressed culinary mustard oil, common in South Asian cooking, is far weaker. Do not confuse the two.
Mustard plasters were a fixture of American home medicine in the 1800s and early 1900s. Nearly every household manual described one.
Eclectic physicians, who worked mainly with plant medicines, listed black mustard as a rubefacient. That word simply means something that reddens skin.
They applied it to the chest for coughs and croup, to the back for lung trouble, and to joints and the lower back for pain. Mustard foot baths were used to warm people who had been out in the cold.
Mustard water was also swallowed to force vomiting after a poisoning. Poison centres abandoned that practice long ago, and modern advice is to call for help instead.
The plaster faded from medicine as aspirin, antibiotics and inhalers arrived. Hospital case reports of mustard burns continued well into recent decades, often in elderly patients.
This is a record of what people did, not a recommendation to repeat it.
Take the plaster off straight away if the pain stops being a sting and becomes a burn.
See a doctor the same day if you have blisters, an open sore, skin that looks white, grey or leathery, or pain that keeps getting worse after removal.
Get urgent care for spreading redness, pus, swelling or fever, which can mean infection.
Call emergency services for wheezing, chest tightness or trouble breathing after exposure to the fumes, or if mustard oil is swallowed or gets in an eye.
See a doctor rather than reaching for mustard if a cough lasts more than three weeks, if you cough up blood, or if chest pain or breathlessness comes on.
In the United States mustard seed is an ordinary food and spice, and is also sold as a supplement. Medicinal plasters are no longer standard pharmacy items. In the UK and EU the seed is a regulated food allergen and must be declared on labels.
Evidence grades and traditional-use labels are explained in our methodology. Nothing here is medical advice.
What it is typically used for, usual dose, cautions, and the study behind it where real trial data exists.
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