There are three subspecies of V. berus that are recognised as being valid including the nominotypical subspecies.

The subspecies V. b. bosniensis and V. b. sachalinensis have been regarded as full species in some recent publications.^{[3][page needed]}

The name 'adder' is derived from nædre, an Old English word that had the generic meaning of snake in the older forms of many Germanic languages. It was commonly used in the Old English version of the Christian Scriptures for the devil and the serpent in the Book of Genesis.^[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 keeping with its wide distribution and familiarity through the ages, Vipera berus has a large number of common names in English, which include:

Common European adder,^{[3][page needed]} common European viper,^[4] European viper,^[12] northern viper,^[13] adder, common adder, crossed viper, European adder,^[10] common viper, European common viper, cross adder,^[9] or common cross adder.^[14]

In Welsh, it is called gwiber, a name derived from Latin vīpera. In Denmark, Norway and Sweden, the snake is known as hugorm, hoggorm and huggorm, roughly translated as 'striking snake'. In Finland, it is known as kyykäärme or simply kyy, in Estonia it is known as rästik, while in Lithuania it is known as angis. In Poland the snake is called żmija zygzakowata, which translates as 'zigzag viper', due to the pattern on its back.

Relatively thick-bodied, adults usually grow to 60 cm (24 in) in total length (including tail), with an average of 55 cm (22 in).^{[3][page needed]} Maximum size varies by region. The largest, at over 90 cm (35 in), are found in Scandinavia; specimens of 104 cm (41 in) have been observed there on two occasions. In France and Great Britain, the maximum size is 80–87 cm (31–34 in).^{[3][page needed]} Mass ranges from 50 g (1.8 oz) to about 180 grams (6.3 oz).^[15][16]

The head is fairly large and distinct and its sides are almost flat and vertical. The edge of the snout is usually raised into a low ridge. Seen from above, the rostral scale is not visible, or only just. Immediately behind the rostral, there are two (rarely one) small scales.

Dorsally, there are usually five large plates: a squarish frontal (longer than wide, sometimes rectangular), two parietals (sometimes with a tiny scale between the frontal and the parietals), and two long and narrow supraoculars. The latter are large and distinct, each separated from the frontal by one to four small scales. The nostril is situated in a shallow depression within a large nasal scale.

The eye is relatively large—equal in size or slightly larger than the nasal scale—but often smaller in females. Below the supraoculars are six to 13 (usually eight to 10) small circumorbital scales. The temporal scales are smooth (rarely weakly keeled). There are 10–12 sublabials and six to 10 (usually eight or 9) supralabials. Of the latter, the numbers 3 and 4 are the largest, while 4 and 5 (rarely 3 and 4) are separated from the eye by a single row of small scales (sometimes two rows in alpine specimens).^[3]

Midbody there are 21 dorsal scales rows (rarely 19, 20, 22, or 23). These are strongly keeled scales, except for those bordering the ventral scales. These scales seem loosely attached to the skin and lower rows become increasingly wide; those closest to the ventral scales are twice as wide as the ones along the midline. The ventral scales number 132–150 in males and 132–158 in females. The anal plate is single. The subcaudals are paired, numbering 32–46 in males and 23–38 in females.^{[3][page needed]}

The colouration of this species exhibits significant variation. Light-coloured specimens are characterised by small, incomplete, dark dorsal crossbars, while darker specimens display faint or clear, darker brown markings. Melanistic individuals are entirely dark in colour and lack any apparent dorsal pattern. However, most specimens show a zigzag dorsal pattern that extends along the entire length of their bodies and tails. A distinctive dark V or X marking is usually present on the dorsal surface of the head. A dark streak runs from the eye to the neck and continues as a longitudinal series of spots along the flanks.^{[3][page needed]}

Unusually for snakes, the sexes can often be distinguished by colour. Females are usually brownish with dark brown markings, while males are pure grey with black markings. The basal colour of the males is often slightly lighter than that of the females, making the black zigzag pattern stand out. Melanistic individuals are often females.

Vipera berus has a wide range. 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. It is found farther north than any other snake species.^{[citation needed]} The type locality was originally listed as 'Europa'. Mertens and Müller (1940) proposed restricting the type locality to Uppsala, Sweden^[2] and it was eventually restricted to Berthåga, Uppsala by designation of a neotype by Krecsák & Wahlgren (2008).^[17]

In several European countries, it is notable for being the only native venomous snake. It is one of only three snake species native to Britain. The other two, the barred grass snake and the smooth snake, are non-venomous.^[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, woodland edges, sunny glades and clearings, scrubby slopes and hedgerows, rubbish tips, coastal dunes, and stone quarries. It will venture into wetlands if dry ground is available nearby, and may therefore be found on the banks of streams, lakes, and ponds.^[19]

In much of southern Europe, such as southern France and northern Italy, it is found in either low lying wetlands or at high altitudes. In the Swiss Alps, it may ascend to about 3,000 m (9,800 ft). In Hungary and Russia, it avoids open steppeland; a habitat in which V. ursinii is more likely to occur. In Russia, however, it does occur in the forest steppe zone.^[19]

In Great Britain, the killing, injuring, harming or selling of adders is illegal under the provisions of the Wildlife and Countryside Act 1981.^[20] A similar situation exists in Norway under the Viltloven [no] (The Wildlife Act 1981)^[21] and in Denmark (1981).^[22] In Finland (Nature Conservation Act 9/2023) killing an adder is legal if it is not possible to capture and transfer it to another location^[23] and the same provision also applies in Sweden.^[24] The common viper is categorised as 'endangered' in Switzerland,^[25] and is also protected in some other countries in its range. It is also found in many protected areas.^[1]

This species is listed as protected (Appendix III) under the Berne Convention.^[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.^[1] A citizen science based survey in the UK found evidence of extensive population declines in the UK, especially affecting smaller populations.^[28] A combination of public pressure and disturbance, habitat fragmentation and poor habitat management were considered the most likely causes of the decline. The release of 47 million non-native pheasants and 10 million partridges each year by countryside estates has also been suggested to have a significant impact on adder populations across the UK, with the possibility the reptile could be extinct by 2032.^[29]

This species is mainly diurnal, especially in the north of its range. Further south it is said^[30] to be active in the evening, and it may even be active at night during the summer months. 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.^[19]

Adders are not usually aggressive, tending to be rather timid and biting only when cornered or alarmed. People are generally bitten only after stepping on them or attempting to pick them up. They will usually disappear into the undergrowth at a hint of any danger, but will return once all is quiet, often to the same spot. Occasionally, individual snakes will reveal their presence with a loud and sustained hissing, presumably to warn off potential aggressors. Often, these turn out to be pregnant females. When the adder is threatened, the front part of the body is drawn into an S-shape to prepare for a strike.^[19]

The species is cold-adapted and hibernates in the winter. In Great Britain, males and females hibernate for about 150 and 180 days, respectively. In northern Sweden hibernation lasts 8–9 months. On mild winter days, they may emerge to bask where the snow has melted and will often travel across snow. About 15% of adults and 30–40% of juveniles die during hibernation.^{[3][page needed]}

Their diet consists mainly of small mammals, such as mice, rats, voles, and shrews, as well as lizards. Sometimes, slow worms are taken, and even weasels and moles. Adders also feed on amphibians, such as frogs, newts, and salamanders. Birds are also reported^[31] to be consumed, especially nestlings and even eggs, for which they will climb into shrubbery and bushes. Generally, diet varies depending on locality.^[19]

Juveniles will eat nestling mammals, small lizards and frogs as well as worms and spiders. One important dietary source for young adders is the alpine salamander (Salamadra atra). Because both species live at higher altitudes, S. atra could be a prevalent food source for adders, since there may be few other animals. One study suggests that alpine salamanders could consist of almost half of the adders' diets in some locations. They have been witnessed swallowing these salamanders in the early morning hours.^[32] Once they reach about 30 cm (0.98 ft) in length, their diet begins to resemble that of the adults.^{[3][page needed]}

In Hungary, mating takes place in the last week of April, whilst in the north it happens later (in the second week of May). Mating has also been observed in June and even early October, but it is not known if this autumn mating results in any offspring.^{[3][page needed]} Females often breed once every two years,^[19] or even once every three years if the seasons are short and the climate is not conducive.^{[3][page needed]}

Males find females by following their scent trails, which can extend over hundreds of metres a day. If a female is found and then flees, the male will pursue her. Courtship involves a synchronised display of a side-by-side 'flowing' movement, tongue flicking along the back and excited tail whipping. Pairs remain together for a day or two following mating. Males expel their rivals and engage in combat. This often begins with the aforementioned flowing behaviour before culminating in the dramatic 'adder dance'.^{[3][page needed]} In this act, the males face each other, raise the front of their bodies vertically, make swaying movements and attempt to push each other to the ground. This sequence of actions is repeated until one of the two males becomes exhausted and withdraws to find another mate. Appleby (1971) notes that he has never seen an intruder win one of these contests, suggesting that the defender's frustration is so intense as a result of the courtship that he refuses to accept a loss in pursuit of mating opportunities.^[33] There is no record of any biting taking place during these bouts.^[19]

Females usually give birth in August or September, but sometimes as early as July, or as late as early October. Litters range in size from 3 to 20. The young are usually born encased in a transparent sac from which they must free themselves. Sometimes, they manage to break free of this membrane while still inside the female.

Neonates measure 14 to 23 cm (5.5 to 9.1 in) in total length (including tail), with an average total length of 17 cm (6.7 in). They are born with a fully functional venom apparatus and a reserve supply of yolk within their bodies. They shed their skins for the first time within a day or two. 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.^[19]

Due to the rapid rate of human expansion throughout the range of this species, bites are relatively common. Domestic animals and livestock are frequent victims. In Great Britain, most cases occur between March and October. In Sweden, there are about 1,300 bites per year, with an estimated 12% requiring hospitalisation.^{[3][page needed]} At least eight different antivenoms are available against bites from this species.^[34]

Mallow et al. (2003) describe the venom toxicity as being relatively low compared to other viper species. They cite Minton (1974) who reported the LD₅₀ values for mice to be 0.55 mg/kg IV, 0.80 mg/kg IP and 6.45 mg/kg SC. As a comparison, in one test the minimum lethal dose of venom for a guinea pig was 40–67 mg, but only 1.7 mg was necessary when Daboia russelii venom was used.^{[3][page needed]} Brown (1973) gives a higher subcutaneous LD₅₀ range of 1.0–4.0 mg/kg.^[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.^{[3][page needed]} In Britain there were only 14 known fatalities between 1876 and 2005—the last a 5-year-old child in 1975^[6]—and one nearly fatal bite of a 39-year-old woman in Essex in 1998.^[6] An 82-year-old woman died following a bite in Germany in 2004, although it is not clear whether her death was due to the effect of the venom.^[35] A 44-year-old British man was left seriously ill after he was bitten by an adder in the Dalby Forest, Yorkshire, in 2014.^[36] Even so, professional medical help should always be sought as soon as possible after any bite.^[37] Very occasionally bites can be life-threatening, particularly in small children, while adults may experience discomfort and disability long after the bite.^[6] The length of recovery varies, but may take up to a year.^{[3][page needed][38]}

Local symptoms include immediate and severe pain, followed after a few minutes (but perhaps up to 30 minutes) by swelling and tingling. Blisters containing blood are not common. Pain may then spread within a few hours, along with tenderness and inflammation. Reddish lymphangitic lines and bruising may appear, and the whole limb can become swollen and bruised within 24 hours. Swelling may also spread to the trunk and, in children, the whole body. Necrosis and intracompartmental syndromes are very rare.^[6]

Systemic symptoms arising from anaphylaxis can be severe. These may manifest within five minutes of the bite, or can be delayed for several hours. These symptoms may include nausea, retching and vomiting, abdominal colic and diarrhoea, incontinence of urine and faeces, sweating, fever, vasoconstriction, tachycardia, lightheadedness, loss of consciousness, and even blindness,^{[citation needed]} shock, angioedema of the face, lips, gums, tongue, throat and epiglottis, urticaria and bronchospasm. If left untreated, these symptoms may persist or fluctuate for up to 48 hours.^[6] In severe cases, cardiovascular failure may occur.^{[3][page needed]}

It was once thought that adders were deaf, as referenced in Psalm 58 (v. 4), yet snake oil derived from adders was used as a cure for deafness and earaches. Females were also thought to swallow their young when threatened and regurgitate them unharmed later. It was further believed that they did not perish until sunset.^[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. Adders were thought to be attracted to hazel trees and repelled by ash trees.^[5]

The Druids believed that large, frenzied gatherings of adders took place in spring, at the centre of which was a polished rock called an adder stone or Glain Neidr in the Welsh language. These stones were said to have held supernatural powers.^[5]