Besides their dramatic flavor, Japanese pickled plums have remarkable medicinal qualities. Their powerful acidity has a paradoxical alkalinizing effect on the body, neutralizing fatigue, stimulating the digestion, and promoting the elimination of toxins. This is the Far Eastern equivalent to both aspirin and apple; not only is it a potent hangover remedy for mornings after; more than that, an umeboshi a day is regarded as one of the best preventive medicines available.

Like many of Japan's ancient medicinal foods, the origin of the pickled plum is obscure. One theory traces it to China, where a dried smoked plum, or ubai, was discovered in a tomb built over two thousand years ago. The ubai is one of China's oldest medicines and is still used for a variety of medical purposes such as counteracting nausea, reducing fevers, and controlling coughs.

The oldest Japanese record of pickled plums being used as a medicine is in a medical text written about one thousand years ago. Umeboshi were used to prevent fatigue, purify water, rid the body of toxins, and cure specific diseases such as dysentery, typhoid, and food poisoning. Slowly, extensive folklore developed about umeboshi's ability to prevent and cure certain diseases.

During Japan's furious samurai period, which lasted through most of the Middle Ages, the pickled plum was the soldier's most important field ration. It was used to flavor foods such as rice and vegetables, and its high acidity made it an excellent water and food purifier, as well as an effective antidote for battle fatigue.

Almost 200 years ago, the Japanese began experimenting with ways to concentrate the healing powers of umeboshi. Finally, a dark liquid called bainiku ekisu (plum extract) was developed. To make the extract, sour green ume plums are slowly cooked down to obtain their most active ingredients in a highly concentrated form. The resulting dark, sticky, thick liquid is usually mixed with hot water and honey and is drunk as a tonic. Dried plum extract is also formed into pills, called meitan. In both plum extract and meitan, the plums' citric acid content is concentrated tenfold, which is equivalent to about twenty-five times the content found in lemon juice.

Many natural healers around the world feel that these concentrated forms of Japanese plums are among the world's most effective natural medicines. Moreover, they do not have the high salt content of pickled plums.