Successful hybridisation between endemic Macrovipera schweizerii from Milos and adjacent islands reported in Kamelin et al. (1997) and results of a recent comprehensive mitochondrial phylogeny of European vipers (Freitas et al. 2020) support suggestions that this species is conspecific with M. lebetinus. These results are however preliminary and no formal taxonomic change has yet been proposed (Speybroeck et al. 2020, W. Wüster pers. comm. 2022) and they are retained as separate species at present.

This species is endemic to Europe, where it is restricted to four of the western Cyclades Islands in Greece. It is assessed as Endangered (EN B1ab(iii)+2ab(iii)) because its extent of occurrence is less than 900 km² and the area of occupancy (AOO) (based on the maximum possible AOO of these four small islands, is less than 500 km². Each of the four small islands is considered to represent one or (in the case of Milos) two locations (resulting in four or five total threat-defined locations), and there is a continuing decline in the extent and quality of its habitat projected as a result of planned mining and quarrying operations.

This species is endemic to the western Cyclades Islands (Milos, Sifnos, Kimolos and Polyaigos), Greece. It is found from sea level up to 500 m asl.

Extrapolating the results of a mark-recapture study between 1994 and 1996 to the area of suitable shrubland on Milos, Nilson (2019) estimated a maximum population size in western Milos of around 3,000 snakes, and a likely total of "slightly more than 2,000 adult vipers" during this survey, noting that this was based on a density estimate (0.5 individuals/ha) up to two orders of magnitude lower than those recorded for island vipers elsewhere in the world. The total number of adult vipers across the entire island is estimated at 2,500 (Böhme et al. 2009, Nilson 2019). Comparable data are lacking from other islands, but an earlier IUCN Red List assessment estimated a maximum population of perhaps 3,000 mature individuals, the majority on the large island Milos (Böhme et al. 2009). In the same period, the assessment for the Greek Red Data Book estimated the total population of the species as “lower than 5,400 individuals” (Ioannidis 2009b).

In 2008, the population of this species on western Milos was estimated at 1,900 individuals based on an extensive mark-recapture study, and the total population on Milos at fewer than 3,000 individuals (Ioannidis 2009a). In the same report, there are data that show a decrease in population density in areas that were also studied between 1994 and 1996. During the 2014-2015 National Monitoring Project, there was a lower “observation index” for Kimolos and Milos, which suggests a decline in population density in comparison with the previous studies.

Assuming 10% survival until adulthood (an estimate calculated for other some other viper species) and 10 eggs per clutch, approximately 25% of the reproductive population is expected to represent newly mature adults (Nilson 2019). Overall, Nilson (2019) estimates that the combined impacts of road mortality, offtake for the pet trade, and deliberate persecution kill 500–600 animals per year and that annual recruitment is about 625 new adults, indicating that the population is likely to be stable only in the absence of other pressures.

It can be found throughout the islands on dry, sunny hillsides and in traditionally cultivated land, in densely vegetated areas close to water with rocky outcrops, and also in deep pools of water. It favours maquis shrubland where large bushes are present, with most animals found during a 15-year radio-tracking study near or beneath vegetation (Nilson 2009). It is likely to occur in lower densities in somewhat degraded 'connecting' habitat corridors than in "large bush" shrubland (Nilson 2019). Radio-tracked individuals on Milos generally had home ranges between 10 and 20 ha, which typically extended to the water source along the valley floor, although animals spent most of their time on slopes away from the streambed outside the spring and autumn foraging seasons (Nilson 2019). It is crepuscular and nocturnal in warmer parts of the year (with average night temperatures around 20°C), and inactive during the daytime when average temperatures are approximately 30°C (Nilson 2019).

Snakes feed primarily on passerine birds, which occur in large numbers on Milos only during twice-yearly migrations, in spring and autumn (Nilson 2019).

The female lays a clutch of between 4-11 eggs. Captive breeding suggests that clutch size is typically equal to or below 10 (Nilson 2019). While longevity is not reported by this author, whose radio tracking project lasted 15 years, he mentions an observation of one female that was found in "exactly the same spot" 10 years after it was first recorded, and a male (marked as adult) that was recaptured 11 years later.

It is threatened by illegal collection of animals, direct persecution when encountered, accidental mortality on roads and ongoing habitat loss through mining and quarrying concessions. Although a bounty system ended in 1977, animals are still killed in "significant numbers" (Nilson 2019). Opencast mining is ongoing on Milos and Kimolos and more mining and quarrying developments are planned (Nilson 2019). Traffic servicing these operations are the primary cause of road mortality, which is likely to be the primary threat to this species (Nilson 2019). Adult vipers make extensive use of roads during summer and autumn (with almost half of observations from roads), and their activity period is strongly correlated with observations of road mortality (Nilson 2019). As many as 100 dead vipers have been found along a single stretch of road in three months following the mating period, and annual mortality on the entire island between 1993 and 2006 is estimated to have averaged 10% of the total population per year (Nilson 2019). Road mortality decreased by almost 50% over the period 2005–2009 after an agreement with the mining company to stop the traffic of lorries during the night in the summer and a pilot project for the construction of barriers and underpasses in key areas of the road network (Ioannidis et al. 2007a, Ioannidis 2009a). The ongoing development of tourism infrastructure, mainly on eastern Milos and Sifnos, is associated with loss of habitat for the species and increased persecution. There are also some plans for tourism development on western Milos.

The viper is expected to be strongly reliant on the passerine migrations that supply its primary food source (Nilson et al. 1999). In years with especially cold autumns or springs vipers may be unable to feed during the major period of migration (Nilson et al. 1999). As the viper population presently appears to exist in a fragile equilibrium state, the loss of body condition or reproductive output in poor years may exacerbate the impacts of ongoing threats.

Fire is not uncommon in vipers' habitat. It can destroy the structure of habitat and turn the area unsuitable for the species for periods that in certain cases exceed 10 years (Ioannidis 2009b). There is an increased threat of wildfires in the more important habitats for the species. Fire is the main threat to the species on the uninhabited island of Polyaigos.

This species was formerly heavily collected for the pet trade, with historical estimates of at least 1,000 individuals, and perhaps up to 1,500, removed from the population annually (Stubbs 1985, Dimitripoulos 1992). This activity has declined but has not fully ceased despite legal protection for the snake, and ongoing annual offtake for the pet trade is estimated at approximately 100 per year (Nilson 2019).

The species is listed in the revised Annex I (species requiring specific habitat conservation measures) and Annex II (strictly protected fauna species) of the Bern Convention, and is in both Annex II (species of community interest whose conservation requires the designation of special areas of conservation) and IV (species of community interest in need of strict protection) of the EU Habitat Directive (in both cases as Vipera lebetina schweizeri). It is protected by national legislation. The entire island of Polyaigos, the larger part of western Milos, the northwestern Kimolos, and a small area of Sifnos are protected within the European network of protected areas.

The pilot construction of road tunnels to limit mortality was largely successful, with daily monitoring finding that vipers used systematically the underpasses and not the road surface in areas where barriers and tunnels are available (Ioannidis et al. 2007b, Ioannidis 2009a), and it is recommended to expand this approach to other parts with high mortality and to any future improvement of road surface.

The restoration of an old inactive mine in western Milos has added some area of suitable habitat for the species (Ioannidis et al. 2005).