Onions are hot and pungent. Onions help prevent blood platelet aggregation, have a cleansing effect upon the lymphs and warm the kidney's yang. They contain anti-cancer phytochemicals including cepaene, disulfides, trisulfides and quercitin. They are considered a supreme food in preventing stomach and skin cancer and tumor growth. They also contain small amounts of prostaglandins A1 and E, which help lower high blood pressure. Onions clear phlegm and respiratory congestion, and help prevent heart attack and stroke. They are used to treat anxiety, flu, atherosclerosis, asthma, blood clots, bronchitis, catarrh, colds, coughs, diabetes, hay fever, hypoglycemia, high cholesterol, high blood pressure, hoarseness, insomnia, obesity, parasitic invasion, pneumonia, rheumatism, sinus congestion, tuberculosis, and urinary infections.

Onions are antibiotic, antiseptic, anti-inflammatory, antioxidant, antiviral, diuretic, expectorant, laxative, liver stimulating, sedative, and tonic. They contain beta-carotene, vitamin B, C, potassium, selenium, and sulfur.

Onions can be strong or mild, with white being the mildest. Red and yellow onions are higher in antioxidants. Look for firm onions with a dry skin, and free of soft spots. Cooking onions deactivates their medicinal properties. Store onions in a cool, dry place. Their volatile sulfurous oil is what causes tears. Peeling them under cold running water prevents the vapors from rising to one's eyes. Enjoy onions in salads, dressings, vegetable dishes, and soups. Banish onion breath by eating some parsley sprigs.

Some may find onions cause flatulence or have irritating action on the bowel. Bee and wasp stings have long been relieved with a poultice of raw onion. Rubbing onion on a balding scalp was a traditional remedy to treat baldness.