Rumex acetosella, also known as sheep sorrel, is a perennial herb, sometimes used as a salad green, and in herbal medicine. Depending on the specific cultivar, the leaves may contain enough oxalic acid to be toxic when eaten in large quantities.
It is an ingredient in herbal cancer treatments, Essiac and FlorEssence, which has shown activity in some small trials and none in others; both had inhibitory action, in vitro, on breast cancer but not leukemia.  According to Memorial Sloan-Kettering Cancer Center, no studies of sheep sorrel alone, as an antineoplastic agent, have been done.
It does, however, contain several classes of biologically active substances:
- Glycosides: Hyperoside, quercitin-3d-galactoside
- Anthraquinones: Emodin, aloe emodin, chrysophanol, rhein, physcion
- Vitamins: A, B complex, C, D, E, K
- Tai J, Cheung S, Wong S, Lowe C (February 2004), "(Abstract) In vitro comparison of Essiac and Flor-Essence on human tumor cell lines.", Oncol Rep 11 (2): 471-6
- Sheep Sorrel, Memorial Sloan-Kettering Cancer Center