Rhamnus purshiana (cascara buckthorn, cascara, bearberry, and in the Chinook Jargon, chittem; syn. Frangula purshiana, Rhamnus purshianus) is a species of buckthorn native to western North America from southern British Columbia south to central California, and inland to western Montana.
Cascara is a large shrub or small tree 4.5–10 m tall, with a trunk 20–50 cm in diameter. The leaves are simple, deciduous, alternate, clustered near the ends of twigs. The flowers are tiny, 4–5 mm diameter, with five greenish yellow petals, forming a cup shape.
The chemicals primarily responsible for the laxative action are the hydroxyanthracene glycosides, which includes cascarosides A,B,C, and D. Cascara contains approximately 8% anthranoids by mass, of which about two-thirds are cascarosides. The hydroxyanthracene glycosides act as a stimulant laxative by exciting peristalsis in the colon.