Medicament reconstitution bag, intended to contain a volume of a liquid substance to be administered to a patient through injectable route, said bag comprises two routes of access, one called input door and the other called output door, the input door of said bag incorporates one filtering element, laid so as to retain any particle forbidden to enter the bag, as well as an input obturating device which may be selectively open or closed, said input door fitted with retention pins to receive a bottle previously fitted with a safety device which allows the bottle fitted with the safety device to be attached irreversibly to the input door; the same input door is fitted with an input breakage region associated to the input obturating device which must be effectively broken to allow its opening; said input door is fitted with an input lid which protects it and covers both a spike as well as the retention pins, said input lid is fitted with an input ring, which links it to the structure of said input door, said input lid is jointed to its input ring by an input weakening line which must be broken to allow the input lid separation; the bag in question, as regards its output door is also fitted with an output obturating device which may selectively be open or closed, said output door is fitted further with an output breakage region associated to the output obturating device which must be effectively broken to allow its opening; said output door is fitted also with an output lid which protects it and which is fitted with an output ring, which links it to the structure of said output door, said output lid is jointed to its output ring by an output weakening line which must be broken to allow the output lid separation, characterized in that the bag, as regards its input door, is fitted with an input tube provided in the lower edge of the bag structure, said input tube receives the assembly of an input tube sector which constitutes a full extension of the obturating device, which is complemented by an input mobile connection; the input obturating device is fitted with an input central projection linked to the structure of said input obturating device by a set of input radial arms, said input central projection is basically assembled internally in an input tubular sector, which is an extension of the input tube sector and serves as means of assembly for the input mobile connection, both the input obturating device and the input mobile connection are components designed to allow mutual assembly of the same as long as the input tubular sector of the input obturating device is sized to fit inside the input mobile connection, remaining between an input tubular and outlining wall of the input mobile connection and an input internal projection equally of tubular profile; the input mobile connection is internally fitted with an input closure wall, which is centrally fitted with an input circular opening sized in its diameter to be fully obstructed by an end of the input central projection incorporated to the input obturating device; the input mobile connection presents an input tubular extension which flows into a chamber occupied by the filtering element unit, said chamber presents a route of passage which continues in a tubular body which configures the spike structure; the input mobile connection structure where is internally arranged the filtering element presents a basically circular configuration, whereas the retention pins mutually in parallel emerge from a lower face of said structure, each one incorporating in its free end a crack-like terminal; the input ring presents input outlining edges, which are sized to produce a tight grasp of the same around the circular structure of the input mobile connection; the input obturating device presents also a first input annular edge which is sized to maintain an overlaying position in relation to a second input annular edge incorporated to the upper end of the input mobile connection, creating thus an input perimetric line which can be welded or jointed to configure the input breakage region which operates as a seal to ensure the bag inviolability; input means of interference are provided between the input mobile connection and the input obturating device, such input means of interference represented by a first input annular projection which integrates the input obturating device and a corresponding second input annular projection, which integrates the input internal tubular projection, said first and second input annular projections are sized and positioned to prevent the total separation of said input obturating device and said input mobile connection, limiting further the mutual displacement between both; wherein the filtering element only touches the liquid substance contained inside the bag upon effective opening of its input door, whereas such filtering element is kept fully isolated and hermetically separated from the internal environment of the bag until effective opening of the input door.