Queen’s flower (banaba)

The queen’s flower, better known by its Philippine name banaba, is a tropical tree of the loosestrife family (Lythraceae). It is native from India and Sri Lanka through Indochina and the Malay Archipelago to the Philippines. Its large mauve to purple flowers earned it names like pride of India. The large, leathery leaves are brewed as a household tea, most famously in the Philippines, where banaba tea is a classic folk preparation connected with blood sugar and with kidney and bladder complaints.

Compared with European herbs, the written record is young, and the first study on blood sugar came from the Philippines in 1940. Modern interest centres on corosolic acid, a plant compound, and on the leaf’s tannins. Many commercial extracts are standardised to contain a stated proportion of corosolic acid. Banaba is sold as a food supplement ingredient in the EU. No health claim has been authorised for it. Human studies are few, and several are very small. If you take blood sugar medication, talk to your doctor before you try banaba.

Banaba’s story belongs to tropical Asia. In the Philippines, boiled leaf tea is a well-known household remedy for diabetes and kidney and bladder complaints. Roots, bark and flowers had folk uses of their own. For India and Bangladesh, where the tree is called jarul, a review records boiled leaf and dried fruit for diabetes. A Thai survey of local healers in 2015 documents diabetes use. Yue’s 2024 review also describes uses in Vietnam and Malaysia, where the country attribution and supporting sources are less secure.

An Indian medicinal plant book already records folk uses of the root, bark, leaves and flowers in 1918. The first published study came from Francisco Garcia, who reported in the Journal of the Philippine Medical Association in 1940 that a decoction of the leaves lowered blood sugar in rabbits for four to six hours. In 1951 Eduardo Quisumbing recorded the folk uses in his compendium Medicinal Plants of the Philippines. From the 1990s Japanese researchers analysed the leaf’s tannins, banaba tea became a popular health drink in Japan, Taiwan and the United States, and leaf extracts standardised to corosolic acid entered the global supplement market. The household tradition is still alive.

The best-studied constituent of the banaba leaf is corosolic acid, a triterpene acid, a plant compound with five rings in its molecular structure also found in loquat, cranberry and several other plants. Commercial extracts are standardised to it, and older leaves hold the most, especially red ones about to drop. The leaf is also rich in water-soluble tannins typical of the species, among them lagerstroemin, flosin B and reginin, the unusual lagerstannin C and valoneaic acid dilactone. The last of these slows a starch-splitting enzyme in the test tube. Corosolic acid dissolves poorly in water, while the tannins pass readily into hot water, so a cup of tea and a capsule standardised to corosolic acid deliver different mixtures.

Almost everything known about how banaba might work comes from cells and animals. There, corosolic acid and tannins increased glucose uptake into cells. In diabetic mice, corosolic acid moved GLUT4, a protein that carries glucose into cells, to the cell surface. In cell lines corosolic acid strengthened insulin receptor signalling. In mice, corosolic acid slowed the breakdown of table sugar in the gut. Banaba extract inhibited enzymes that split starch and sugar in laboratory tests. In isolated rat livers, corosolic acid reduced the production of new glucose. Mouse studies also report changes in PPAR, a group of receptors that steer fat and sugar metabolism. Some of these effects need concentrations that a cup of tea or a capsule is unlikely to reach in the body. In laboratory tests purified corosolic acid also slowed the drug-metabolising enzyme CYP3A, and banaba extract slowed SULT1A3, another enzyme that processes medicines. These laboratory findings are why the safety section asks for care with some medicines.

Human studies of banaba on its own are few, and several are very small. In 2003 Judy and colleagues gave ten people with type 2 diabetes 32 or 48 mg a day of a leaf extract standardised to 1 percent corosolic acid for two weeks. In this small randomised study, blood glucose fell by up to about 30 percent. In 2006 Fukushima and colleagues gave 31 people 10 mg of purified corosolic acid on one occasion and placebo on another, each time before a glucose drink. Neither the volunteers nor the researchers knew which capsule was which. From 60 minutes on, glucose ran lower with corosolic acid, with a statistically significant difference at 90 minutes. In a one-year Japanese study, 15 people took a water-soluble leaf extract, and everyone knew what they were taking. In those who started above 110 mg per dL, fasting glucose was 16.6 percent lower, with no side effects and no episodes of low blood sugar. In 2014 Choi and colleagues compared banaba extract, soybean leaf extract and placebo in separate groups for twelve weeks. The banaba group took 300 mg extract with 0.3 percent corosolic acid daily. After accounting for starting values, blood glucose was lower than with placebo. Body weight did not differ between groups.

López-Murillo and colleagues’ 2022 trial assigned treatments at random and compared banaba with a dummy preparation (placebo). Neither participants nor researchers knew which treatment was given. In it, 24 adults with metabolic syndrome, a group of health problems that raises the risk of diabetes and cardiovascular disease, took 500 mg of banaba or placebo twice daily for twelve weeks, with twelve people in each group. In the banaba group fasting glucose fell from 5.9 to 5.7 mmol per litre and triglycerides (blood fats) from 2.3 to 1.7 mmol per litre. The upper blood pressure reading, VLDL (particles that carry fat in the blood) and insulin secretion also dropped, and eight of the twelve people taking banaba no longer met the criteria for metabolic syndrome. These falls were measured against the banaba group’s own starting values. It is one small single-centre trial.

Several often-cited studies tested mixtures, such as the 1999 Banabamin tablets, which combined banaba with green tea, green coffee and Garcinia, so their results cannot be credited to banaba. The two best-known reviews need care too. A 2012 review by Miura and colleagues was retracted in 2013 because it duplicated the review by Stohs and colleagues. The Stohs review, in turn, lists authors with ties to a supplier of banaba extracts.

Forms and preparation

In its home region banaba is a boiled leaf tea. Folk recipes simmer fresh or dried leaves. One recorded version uses about 50 g of old leaves and dried fruit to a pint of boiling water, and four to six cups of this brew are drunk a day. The tannin-rich leaf gives a reddish-brown, slightly astringent cup. These folk recipes are history, not a dosing recommendation. Today banaba is sold as cut leaf and in tea bags and, above all, as extracts standardised to corosolic acid, usually in capsules or soft gels. Studies used extracts with anywhere from 0.3 to 18 percent corosolic acid. Tea and extract deliver different mixtures of compounds, so read the label, and follow its instructions when taking an extract.

Dosage

There is no official dose. Published studies used 32 to 48 mg a day of an extract standardised to 1 percent corosolic acid for two weeks, or a single 10 mg dose of purified corosolic acid. Choi 2014 used 300 mg a day of extract with 0.3 percent corosolic acid for twelve weeks. Other studies used 100 mg a day of a water-soluble extract for one year, or 500 mg of banaba twice daily for twelve weeks. These are study settings, not recommendations. If you take glucose-lowering medication or live with diabetes, talk to your doctor before using banaba at all, because its effects could add to your medication. Otherwise follow the product label, do not exceed it, and stop and seek advice if unusual symptoms appear.

Special populations

No reliable studies cover pregnancy, breastfeeding or childhood, so avoid banaba while pregnant or breastfeeding, and do not give it to children. If your kidneys are impaired, avoid it too, because the described case combined corosolic acid with chronic kidney disease and a painkiller. If you have diabetes, stay under medical care and never swap prescribed treatment for a leaf tea.

Drug interactions

The interaction that matters most follows from why people take banaba. Because it is taken to influence blood glucose, on top of insulin or glucose-lowering tablets it could push glucose too low, and this combination needs medical supervision and closer glucose checks. The published kidney case combined corosolic acid with diclofenac in chronic kidney disease. Do not combine banaba with regular non-steroidal anti-inflammatory painkillers (NSAIDs), such as diclofenac or ibuprofen. If your kidneys are impaired, avoid banaba altogether. In laboratory tests purified corosolic acid slowed the drug-metabolising enzyme CYP3A, and banaba extract slowed SULT1A3, another enzyme that processes medicines, which could raise the levels of some medicines. No such interaction has been confirmed in people, but tell your doctor you take banaba if you use any medicine whose dose or blood level must stay within a small range for safe treatment.

Contraindications

The precautions below rest on the published studies, the kidney case report and missing data for some groups. Allergy to banaba or other Lagerstroemia species. Pregnancy, breastfeeding and children: no safety data, avoid. Existing kidney disease: avoid, given the published case of acute kidney injury and lactic acidosis in a man whose kidneys were already impaired. Insulin or glucose-lowering medication: combine only under medical supervision, because blood sugar could fall too low.

Side effects

The small trials described in the Stohs review, including one year of daily use in 15 people, reported no side effects and no episodes of low blood sugar. A case report describes acute kidney injury with lactic acidosis, excessive acidity in the blood, in a man with impaired kidneys. The case is described under the warnings. Allergic reactions are possible, although the studies checked here did not report them. A safe upper limit for use in people has not been established. If you notice any unwanted reaction, stop taking banaba and ask your doctor or pharmacist.