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Summary

A plant named for the messenger of the gods might be expected to have some special properties but a 'dog's' plant is one with no use.

Family

Euphorbiaceae

Meaning of the Name

Mercurialis
Most sources follow Pliny who said the plant is named after Mercury, the messenger of the gods, who discovered it. The word ‘mercury’ itself is, however, said to be related to ‘merx’ meaning ‘wares’. With its alleged ability to determine the sex of an unborn child, it may be that this plant was traded and was ‘merx’.
 
Medicinally, the ‘annua’ species of the plant was used as a purgative, diuretic or antisyphilitic. Perhaps, as an attractive, athletic man travelling widely to deliver the gods’ messages, Mercury had need of its properties.
 
perennis
Perrenial to distinguish it from the ‘annua’ used medicinally.

Common Names and Synonyms

dog’s mercury, adder’s meat, lasting mercury

How Poisonous, How Harmful?

It contains methylamine, trimethylamine, saponins and a volatile oil. It is emetic and purgative leading to nausea, vomiting and diarrhoea. Larger doses cause lethargy, jaundice, painful urination, apparently by making the urine acid, and coma before death.

There is nothing appealing about the appearance of the plant and it had no medical uses so instances of poisoning are few.

Incidents

In April 2017, a runner out for his morning exercise mistook the plant for ground elder and ate 'a good handful or two'. 'About 3 minutes later (certainly less than 5) I noticed an increase in my saliva. Over the following 3 minutes it grew and was accompanied by an ominous nausea which eventually blossomed into about 2 dry retches and 3 good mouthfuls of vomit. The first contained mostly green saliva. The second, most of the green material and the third my breakfast cereal.' He felt immediately better and was able to complete his run.

In the 1980s, a couple were reported to have eaten the leaves, boiled, thinking it to be a green vegetable. They suffered severe gastrointestinal upset leading to dehydration and were given sodium bicarbonate to neutralise the acidity of the urine. They recovered within two days.

A number of incidents of poisoning in sheep and cattle have been reported.

Folklore and Facts

Mercurialis perennis, dog’s mercury

A 'dog's' plant is one with no medicinal uses and it may have obtained this name to contrast it with annual mercury which was used in cleansing enemas.

In more recent times, the fresh plant has been used as a laxative but there is some evidence that it has a cumulative effect and its use, medicinally, is discouraged even by herbalists. It is though offered by some homeopaths but, of course, in that case there will be none of the actual plant in the remedy.

Pliny talks of two versions of Mercurialis, one male, one female. It is possible he means the M. annua and the M. perennis. Use of the male plant will ensure a male child and the female, female. Hippocrates used both plants for a great many conditions mostly related to the female reproductive system. It formed part of a complex mixture which had chicken as its main ingredient and was used to treat a long list of conditions. Talking of chickens gives Pliny the chance to report that a hen’s leg dipped in molten gold will absorb all of the gold and a collar made of gold shavings round the neck of a cockerel will stop him crowing.