[5] It is not regarded as especially dangerous;[3][page needed] the snake is not aggressive and usually bites only when really provoked, stepped on, or picked up.

It feeds on small mammals, birds, lizards, and amphibians, and in some cases on spiders, worms, and insects.

Females breed once every two or three years, with litters usually being born in late summer to early autumn in the Northern Hemisphere.

Adults grow to a total length (including tail) of 60 to 90 cm (24 to 35 in) and a mass of 50 to 180 g (1.8 to 6.3 oz)[citation needed].

[3][page needed] The name 'adder' is derived from nædre, an Old English word that had the generic meaning of serpent in the older forms of many Germanic languages.

[5][11] In the 14th century, 'a nadder' in Middle English was rebracketed to 'an adder' (just as 'a napron' became 'an apron' and 'a nompere' changed into 'an umpire').

In Poland the snake is called żmija zygzakowata, which translates as 'zigzag viper', due to the pattern on its back.

[3][page needed] The colour pattern varies, ranging from very light-coloured specimens with small, incomplete, dark dorsal crossbars to entirely brown ones with faint or clear, darker brown markings, and on to melanistic individuals that are entirely dark and lack any apparent dorsal pattern.

A dark streak runs from the eye to the neck and continues as a longitudinal series of spots along the flanks.

The basal colour of males will often be slightly lighter than that of the females, making the black zigzag pattern stand out.

It can be found across the Eurasian land-mass; from northwestern Europe (Great Britain, Belgium, Netherlands, Scandinavia, Germany, France) across southern Europe (Italy, Serbia, Albania, Croatia, Montenegro, Bosnia and Herzegovina, North Macedonia, Bulgaria, and northern Greece) and eastern Europe to north of the Arctic Circle, and Russia to the Pacific Ocean, Sakhalin Island, North Korea, northern Mongolia and northern China.

[18] Sufficient habitat complexity is a crucial requirement for the presence of this species, in order to support its various behaviours—basking, foraging, and hibernation—as well as to offer some protection from predators and human harassment.

[3][page needed] It is found in a variety of habitats, including: chalky downs, rocky hillsides, moors, sandy heaths, meadows, rough commons, edges of woods, sunny glades and clearings, bushy slopes and hedgerows, dumps, coastal dunes, and stone quarries.

It will venture into wetlands if dry ground is available nearby and thus may be found on the banks of streams, lakes, and ponds.

[19] In Great Britain, it is illegal to kill, injure, harm or sell adders under the Wildlife and Countryside Act 1981.

[22] In Finland (Nature Conservation Act 9/2023) killing an adder is legal if it's not possible to capture and transfer it to another location[23] and the same provision also applies in Sweden.

[26] The International Union for Conservation of Nature Red List of Threatened Species describes the conservation status as of 'least concern' in view of its wide distribution, presumed large population, broad range of habitats, and likely slow rate of decline though it acknowledges the population to be decreasing.

[27] Reduction in habitat for a variety of reasons, fragmentation of populations in Europe due to intense agriculture practices, and collection for the pet trade or for venom extraction have been recorded as major contributing factors for its decline.

It is predominantly a terrestrial species, although it has been known to climb up banks and into low bushes in order to bask or search for prey.

Occasionally, individual snakes will reveal their presence with a loud and sustained hissing, presumably to warn off potential aggressors.

[3][page needed] Their diet consists mainly of small mammals, such as mice, rats, voles, and shrews, as well as lizards.

[32] Because both species live at higher altitudes, S. atra could be a prevalent food source for adders, since there may be few other animals.

[3][page needed] Males find females by following their scent trails, sometimes tracking them for hundreds of metres a day.

Courtship involves side-by-side parallel 'flowing' behaviour, tongue flicking along the back and excited lashing of the tail.

[3][page needed] In this act, the males confront each other, raise up the front part of the body vertically, make swaying movements and attempt to push each other to the ground.

Appleby (1971) notes that he has never seen an intruder win one of these contests, as if the frustrated defender is so aroused by courtship that he refuses to lose his chance to mate.

Females do not appear to take much interest in their offspring, but the young have been observed to remain near their mothers for several days after birth.

[14] All agree that the venom yield is low: Minton (1974) mentions 10–18 mg for specimens 48–62 cm (19–24.5 in) in length,[3][page needed] while Brown (1973) lists only 6 mg.[14] Relatively speaking, bites from this species are not highly dangerous.

Such symptoms include nausea, retching and vomiting, abdominal colic and diarrhoea, incontinence of urine and faeces, sweating, fever, vasoconstriction, tachycardia, lightheadedness, loss of consciousness, blindness,[citation needed] shock, angioedema of the face, lips, gums, tongue, throat and epiglottis, urticaria and bronchospasm.

[39] Remedies for adder "stings" included killing the snake responsible and rubbing the corpse or its fat on the wound, also holding a pigeon or chicken on the bite, or jumping over water.