The common adder is found in different terrains, habitat complexity being essential for different aspects of its behaviour. It feeds on small mammals, birds, lizards, amphibians and in some cases on spiders, worms and insects. Females breed once every two or three years with litters usually born in late summer to early autumn in the Northern hemisphere. The common adder, like most other vipers, is ovoviviparous; litters range in size from 3 to 20 with young staying with their mothers for a few days. Adults grow to a length of 60 to 90 centimetres (24 to 35 in) and a mass of 50 grams (1.8 oz) to about 180 grams (6.3 oz). Three subspecies are recognized, including the nominate subspecies described here. The snake is not considered to be threatened though it is protected in some countries.
Relatively thick-bodied, adults grow to 60 centimetres (24 in) in length with an average of 55 centimetres (22 in). Maximum size varies per region. The largest, at over 90 centimetres (35 in), are found in Scandinavia; specimens of 104 centimetres (41 in) have been observed there on two occasions. In France and Great Britain, the maximum size is 80–87 centimetres (31–34 in). Mass ranges from 50 grams (1.8 oz) to about 180 grams (6.3 oz).
The head is fairly large and distinct, the sides of which 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 2 (rarely 1) small scales. Dorsally, there are usually 5 large plates: a squarish frontal (longer than wide, sometimes rectangular), 2 parietals (sometimes with a tiny scale between the frontal and the parietals), and 2 long and narrow supraoculars. The latter are large and distinct, each separated from the frontal by 1-4 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 there are 6-13 (usually 8-10) small circumorbital scales. The temporal scales are smooth (rarely weakly keeled). There are 10-12 sublabials and 6-10 (usually 8-9) 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).
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.
The color pattern varies, ranging from very light-colored 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. However, most have some kind of zigzag dorsal pattern down the entire length of the body and tail. The head usually has a distinctive dark V or X on the back. A dark streak runs from the eye to the neck and continues as a longitudinal series of spots along the flanks. Unusual for snakes, the sexes are possible to tell apart by the colour. Females are usually brownish in hue with dark-brown markings, the males are pure grey with black markings. The basal colour of males will often be a tad lighter than that of the females, making the black zigzag pattern stand out. The melanistic individuals are often females.
