It grows erect to a height of 30–110 cm in moist shady places with glabrous leaves and white flowers with rose-purple spots on the petals. Stem dark green, 0.3 - 1.0 m in height, 2 – 6 mm in diameter, quadrangular with longitudinal furrows and wings on the angles of the younger parts, slightly enlarged at the nodes; leaves glabrous, up to 8.0 cm long and 2.5 cm broad, lanceolate, pinnate; flowers small, in lax spreading axillary and terminal racemes or panicles; capsules linear-oblong, acute at both ends, 1.9 cm x 0.3 cm; seeds numerous, sub quadrate, yellowish brown.
Andrographis paniculata can cause severe allergic reactions. Commercial products containing this as a herbal remedy have been recalled in Australia as a result.
Since ancient times, A. paniculata is used in traditional Siddha and Ayurvedic systems of medicine as well as in tribal medicine in India and some other countries for multiple clinical applications. The therapeutic value of Kalmegh is due to its mechanism of action which is perhaps by enzyme induction. The plant extract exhibits antityphoid and antifungal activities. Kalmegh is also reported to possess antihepatotoxic, antibiotic, antimalarial, antihepatitic, antithrombogenic, antiinflammatory, anti-snake venom, and antipyretic properties to mention a few, besides its general use as an immunostimulant agent. A recent study conducted at Bastyr University, confirms the anti-HIV activity of andrographolide.
Andrographolide, the chief constituent extracted from the leaves of the plant, is a bitter water-soluble lactone exhibiting protective effects in carbon tetrachloride induced hepatotoxicity in rats. Its LD50 in male mice was 11.46gm/kg, ip. This bitter principle was isolated in pure form by Gorter (1911). Such other activities as liver protection under various experimental conditions of treatment with galactosamine, paracetamol etc. are also attributed to Andrographolide. The hepatoprotective action of andrographolide is related to activity of certain metabolic enzymes.
Andrographis paniculata plant extract is known to possess a variety of pharmacological activities. Andrographolide, the major constituent of the extract, is implicated in its pharmacological activity. A study has been conducted on the cellular processes and targets modulated by andrographolide treatment in human cancer and immune cells. Andrographolide treatment inhibited the in vitro proliferation of different tumor cell lines, representing various types of cancers. The compound exerts direct anticancer activity on cancer cells by cell cycle arrest at G0/G1 phase through induction of cell cycle inhibitory protein p27 and decreased expression of cyclin dependent kinase 4 (CDK4). Immunostimulatory activity of andrographolide is evidenced by increased proliferation of lymphocytes and production of interleukin 2. Andrographolide also enhanced the tumor necrosis factor α production and CD marker expression, resulting in increased cytotoxic activity of lymphocytes against cancer cells, which may contribute for its indirect anticancer activity. The in vivo anticancer activity of the compound is further substantiated against B16F0 melanoma syngenic and HT 29 xenograft models. These results suggest that andrographolide is an interesting pharmacophore with anticancer and immunomodulatory activities and hence has the potential for being developed as a cancer therapeutic agent. 
In one Chilean study, the herb had a significant drying effect on the nasal secretions of cold sufferers who took 1,200 milligrams of andrographis extract daily for five days.[unreliable source?]
The herb is the well-known drug Kalmegh 'green chiretta', and forms the principal ingredient of a household medicine ('alui'), used as a bitter tonic and febrifuge.
The Tamils have been using Nilavempu - as it is called in Tamil - for centuries. In Siddha medicine, Andrographis Paniculata is used widely to treat fevers like chikenguinea, swine-flu, typhoid etc.[unreliable source?]
Andrographolide is the major constituent extracted from the leaves of the plant which is a bicyclic diterpenoid lactone. Some known constituents are:
• "14-Deoxy-11-dehydroandrographolide, Plant
• 14-Deoxy-11-oxoandrographolide, Plant
• 5-Hydroxy-7,8,2',3'-Tetramethoxyflavone, Plant
• 5-Hydroxy-7,8,2'-Trimethoxyflavone, Tissue Culture
• Andrographine, Root
• Andrographolide, Plant
• Neoandrographolide, Plant
• Panicoline, Root
• Paniculide-A, Plant
• Paniculide-B, Plant
• Paniculide-C, Plant"