A needle assembly (10) comprising: a body (12) with a proximal end (18), a distal end (20) and a channel (22) located between them, said body being configured to be mounted on an injector ( I)； a needle (14) fixed on said body having a proximal end (28) and a distal end (30), said distal end extending distally from said distal end of said body and being formed for insertion into a patient, being said proximal end of said needle communicated with said channel； a filter (16) disposed in the channel proximally in relation to the proximal end of the needle； a second filter (36) disposed in said channel proximally in relation to said filter (16) and separated from it； and a drug substance (32) disposed in said channel (22) proximally in relation to said filter (16) and distally in relation to said second filter (36), the overall porosity of said filter (16) being provided ) and the overall porosity of said second filter (36) so as not to allow said drug substance (32) to pass through said filters； characterized in that said filter (16) is porous and is configured so that liquid located in the channel proximally in relation to said filter can pass through said filter； and wherein said second filter (36) is more porous than said filter (16), so that one or more secondary substances (34) can be passed through said second filter (36) more easily than through the filter (16).Un conjunto de aguja (10) que comprende: un cuerpo (12) con un extremo proximal (18), un extremo distal (20) y un canal (22) situado entre ellos, estando configurado dicho cuerpo para ser montado en un inyector (I)； una aguja (14) fijada en dicho cuerpo que presenta un extremo proximal (28) y un extremo distal (30), extendiéndose dicho extremo distal de manera distal desde dicho extremo distal de dicho cuerpo y estando formado para su inserción en un paciente, estando comunicado dicho extremo proximal de dicha aguja con dicho canal； un filtro (16) dispuesto en el canal de manera proximal en relación con e
