Thebu (Costus speciosus): The "Insulin Plant" and What the Evidence Really Says

Ask any Sri Lankan gardener about thebu and you will hear the same instruction: "one leaf a day for sugar." Thebu (Costus speciosus) has become the island's most popular home remedy for diabetes in the space of a generation, its spiral stems and crepe-paper white flowers now planted beside kitchens across the country. That popularity has run well ahead of the science, and in 2024 the science answered back with a warning worth reading.

Botanical and Cultural Background

Thebu is a rhizomatous perennial of the Costaceae family, recognised by leaves arranged in a distinctive spiral around the stem. Botanists have recently reclassified it as Hellenia speciosa, and a further muddle surrounds the name "insulin plant", which is applied in Sri Lanka to thebu but in India mostly to a close relative, Costus igneus (also called C. pictus). Much of the research quoted in support of thebu actually concerns that Indian cousin. Traditionally the rhizome, not the leaf, was the medicinal part, valued in Ayurveda as a source of diosgenin and used for fevers, skin conditions and urinary complaints.

Traditional Uses in Sri Lankan Medicine

• Leaves — the modern folk use: one or two fresh leaves chewed daily, or leaves added to sambola and mellum, to "control sugar".
• Rhizome — the classical use: decoctions for fever, cough, urinary tract complaints and skin disease; also applied externally for rashes.
• Whole plant — grown around homes partly as an ornamental and partly as a living medicine chest.

What the Research Shows

Laboratory evidence is genuinely encouraging: costunolide isolated from the rhizome lowered blood glucose and lipids in diabetic rats (Eliza et al., 2009), and reviews of the related insulin plants describe leaf compounds that improve glucose uptake, with small human observations suggesting 500–2,000 mg of dried leaf daily may lower blood glucose (Chandrakar et al., 2024). The 2023 literature reviews also confirm hepatoprotective, anti-inflammatory and antioxidant activity in animal models.

The honest caveat is that no properly controlled human trial establishes a safe, effective dose. Worse, the plant works well enough to cause harm when combined with medicines: a 2024 report in Tropical Doctor described two patients on diabetes drugs who developed serious hypoglycaemia after adding insulin-plant leaves to their routine (Kaki & Savino, 2024). Thebu is therefore best understood as a plant with real but unquantified glucose-lowering activity.

Evidence updated September 2026.

How Thebu Is Prepared

The everyday method is simply one washed fresh leaf chewed before breakfast, or two or three leaves finely shredded into a sambola with onion, lime and grated coconut. Leaves can be sun-dried and powdered; a level teaspoon in warm water is the usual measure. Rhizome decoctions belong in the hands of a qualified practitioner.

Safety and Precautions

Do not add thebu to an existing diabetes regimen without telling your doctor and monitoring blood sugar closely; the combination has caused hospital admissions for hypoglycaemia. Anyone experiencing sweating, shakiness or confusion after taking it should eat something sugary and seek help. Avoid in pregnancy and breastfeeding for lack of safety data. Never stop prescribed medication in favour of the leaf.