From WikiPilipinas: The Hip 'n Free Philippine Encyclopedia
LAGERSTROEMIA SPECIOSA also known as BANABA
Banaba is found n the Batan Islands and northern Luzon to Palawan, Mindanao, and the Sulu Archipelago, in most or all island and provinces, chiefly in secondary forest at low and medium altitudes. It is also reported to occur in India to southern China and southward through Malaya to tropical Australia.
Banaba is cultivated in Manila for its beautiful flowers. It makes an excellent avenue tree and very effective when massed in parks. Banaba is also useful as a timber tree.
The old leaves and ripe fruits are the parts of banaba that contain the greatest amount of an insulin-like principle. Twenty grams of old leaves or fruit, dried from one to two weeks, in the from of 100 cc. of 20 per cent decoction were found to have the activity equivalent to form 6 to 7.7 units of insulin in lowering blood sugar.
The mature leaves, young leaves, and flowers have an activity that range from 4.4 to 5.4 units of insulin per 100 cc. of 20 per cent decoction, or equivalent to around 70 per cent of the activity of the leaves or fruit.
The wood does not contain the insulin-like principle while the bark and roots contain a very small amount.
In the Philippines, banaba is popular medicinal plant. A decoction of the leaves of all ages is used for diabetes mellitus. It is prepared and taken like tea. Some Filipino physicians believe that a decoction of the dried fruit is even better.
Kirtikar and Basu quote Dr. Stewart, who considers the bark stimulant and febrifuge. Burkill and Haniff state that a decoction of it is used in Pahang for abdominal pains. Heyne says that an infusion is taken to stop diarrhea. According to Duchesne a decoction of the roots is used against small ulcers of the mouth. He also considers a decoction of the leaves a deobstruent and diuretic. Grin writes that the bark, leaves, and flowers are given as a purgative. The seeds possess narcotic properties and are employed against aphthae.