The strong smell that characterizes the onions is not perceived until they are cut; at that moment, the endogenous enzymes stimulate the formation of a lacrimatory compound called allicine. The allicine is afterwards decomposed in other sulphurous compounds that, apparently, help to control diabetes and the level of lipids in the blood.

It has been demonstrated that the onions, specially those with coloured tissues, are important sources of flavonoids, a kind of compounds with an antioxidant activity related to beneficial effects against cardiovascular diseases and, most probably, cancer. These flavonoids, in the form of glucosides, are extraordinarily stable during storage and cooking. The onions play an important role in keeping a balanced diet.

Among their multiple beneficial properties, onions help in the protection of stomach disorders, like cancer and ulcers. The suitable levels of vitamin B help to maintain the system nervous. Onions have been used with medicinal purposes since the old times.

The onion has been used for a long time as a medicinal plant; that is why there exist many prescriptions and remedies for different aches and pathologies. It is used in many ways, in juice, tinctures, wine, poultice, infusion or roasted.

It is used to fight acne applying clay and onion poultices. It is also useful against fever. In case of throat infection and aphony the onion is macerated with lemon. The onion broth is indicated against cystitis, whereas for colds one recommends onion and lemon syrup. For high blood pressure, mix garlic and onion with lemon juice. If the onions are macerated in dry white wine, they are useful for the good functioning of kidneys.

In order to get rid of toxins, drink an infusion of fresh onions. If they are cooked with honey, they result in a purifying or vermifugal drink. The onion juice is useful for bees and wasps stings, cough, colds, asthma and bronchitis. The onion wine is vermifugal and diuretic. The onion poultices are used in case of very bad headaches, belly inflammations, throat infections, wounds and haemorrhoids.