A reducer applicable to an opening of a container and identifying a concavity (2) adapted to house a portion of a syringe doser. The reducer (1) comprises a bottom (21) of said concavity (2) and a lateral wall (22) 5 that defines a side of said concavity (2). The bottom (21) comprises an opening (210) adapted to allow the passage of a fluid present in the container. The reducer (1) comprises at least a first groove (31) placed in the concavity (2) and afforded at least on said lateral wall (22).
