An arrangement for a drug delivery device, the arrangement comprising: - a cap defining a cavity and - a body, - wherein one of the cap and the body comprises a recess, - wherein the other one of the cap and the body is provided with a seal, the seal comprising a sealing surface, - wherein the cap and the body are configured such that the cap is removably attachable to the body, - wherein, when the cap is attached, the sealing surface of the seal seals a section of the cavity and wherein, - A) when the cap comprises the recess: the recess is arranged on a side of the seal which is remote from the section of the cavity, during detachment of the cap from the body the recess is moved towards the side of the seal which faces the section of the cavity, and, when at least a part of the recess has passed the sealing surface, this part of the recess establishes a passageway between the cavity and the environment - B) when the body comprises the recess: the seal is arranged on a side of the recess which is remote from the section of the cavity, during detachment of the cap from the body the seal is moved towards the side of the recess which faces the section of the cavity and, when the sealing surface has passed at least a part of the recess, this part of the recess establishes a passageway between the cavity and the environment. - a cap defining a cavity and - a body, - wherein one of the cap and the body comprises a recess, - wherein the other one of the cap and the body is provided with a seal, the seal comprising a sealing surface, - wherein the cap and the body are configured such that the cap is removably attachable to the body, - wherein, when the cap is attached, the sealing surface of the seal seals a section of the cavity and wherein, - A) when the cap comprises the recess: the recess is arranged on a side of the seal which is remote from the section of the cavity, during detachment of the cap from the body the recess is moved towards the side of the seal which faces the section of the cavity, and, when at least a part of the recess has passed the sealing surface, this part of the recess establishes a passageway between the cavity and the environment the recess is arranged on a side of the seal which is remote from the section of the cavity, during detachment of the cap from the body the recess is moved towards the side of the seal which faces the section of the cavity, and, when at least a part of the recess has passed the sealing surface, this part of the recess establishes a passageway between the cavity and the environment - B) when the body comprises the recess: the seal is arranged on a side of the recess which is remote from the section of the cavity, during detachment of the cap from the body the seal is moved towards the side of the recess which faces the section of the cavity and, when the sealing surface has passed at least a part of the recess, this part of the recess establishes a passageway between the cavity and the environment. the seal is arranged on a side of the recess which is remote from the section of the cavity, during detachment of the cap from the body the seal is moved towards the side of the recess which faces the section of the cavity and, when the sealing surface has passed at least a part of the recess, this part of the recess establishes a passageway between the cavity and the environment.