Cynanchum is a genus of about 300 species including some swallowworts, belonging to the family Apocynaceae.
Cynanchum atratum Bunge, Herbs, most parts densely pubescent to tomentose. Roots fibrous, fragrant. Stems erect, to 50 cm, sometimes ± inflated. Leaves opposite; petiole ca. 5 mm; leaf blade ovate or ovate-oblong, 5-8(-12) × 3-4(-7) cm, papery, base rounded or sometimes obscurely cordate, apex acute or acuminate; lateral veins 6 or 7 pairs. Inflorescences umbel-like, sessile, 8-10-flowered. Pedicel ca. 1.5 cm.

Eleven new 14,15-seco-pregnane-type steroidal glycosides, cynanosides P1-P5, Q1-Q3, R1-R3, and a novel 12,13-seco-14,18-nor-pregnane-type steroidal glycoside, cynanoside S, were isolated from the roots of Cynanchum atratum, together with four known compounds, atratoside C, sublanceoside E3, chekiangensoside C and cynatroside B. Their structures were determined on the basis of spectroscopic analysis and chemical evidence.