Mercurialis perennis, commonly known as dog's mercury, is a poisonous woodland plant found in much of Europe as well as in Algeria, Iran, Turkey, and the Caucasus, but almost absent from Ireland, Orkney and Shetland.

[1][2] A member of the spurge family (Euphorbiaceae), it is a herbaceous, downy perennial with erect stems bearing simple, serrate leaves.

It usually grows in dense masses often in the ground flora of beech, oak, ash, elm and other types of woodlands in Europe.

[3] Dog's mercury favors alkaline (basic) soils and can be found in abundance in suitable habitats in limestone regions.

Spreading by underground rhizomes, where its dense growth may shade out other woodland flowers such as oxlip, fly orchid, and even young ash seedlings,[4] but in the open, it eventually gives way to other plants.

The ultimate height attained in different mountainous regions, e.g. in Scotland, England, Germany, and Switzerland, naturally varies with the latitude and other geographical factors.

[3] Existing colonies in some parts of Britain (including some in woods on boulder clay in East Anglia), are expanding and showing increased vigor, perhaps as a result of deeper shade in woodlands where coppicing has ceased.

[3] The genus Mercurialis itself consists of nine species and the main taxonomic characteristics used in distinguishing them are the clusters of floration, the annual or perennial habit, and the glabrous or hairy condition of the vegetative organs, but chiefly the ovary and the capsule, the woody or herbaceous nature of the plant, and lastly the character of the lamina.

[3] Dog's mercury is one of the characteristic plants of several woodland types, in particular: M. perennis has variation in its morphological characters.

genuina Miiller-Aarg in the following respects: Besides those three variations of M. perennis there are six habitat forms in nature:[3] All parts of the dog's mercury are highly poisonous.

A researcher induced toxicity with dog's mercury, frozen at different stages of growth and fed it to sheep.

[7] An outbreak of fatal mercurialis poisoning in the Welsh mountain ewes was reported which included hemolytic anemia without marrow suppression and acute oedematous gastroenteritis with hepatic centrilobular necrosis.

[citation needed] A fine blue dye can be obtained from the leaves although it is able to be turned red by acids and destroyed by alkalis.