A cap assembly adapted to be attached to a drug delivery device main unit, comprising a drug-filled cartridge with an axially moveable piston and a distal portion with a needle penetratable seal, a needle assembly and a cap. The needle assembly comprises a needle having a distal skin-piercing end and a proximal seal-piercing end. The cap comprises a reservoir adapted to receive the needle distal end, the cap accommodating the needle assembly and at least the distal portion of the cartridge. The needle assembly and the cartridge are arranged axially moveable relative to each other between an initial state in which the proximal needle end is arranged distally of the cartridge seal, and an operational state in which the proximal needle end has penetrated the cartridge seal, wherein the reservoir is adapted to receive drug from the cartridge via the needle.
