Needle insertion by overpressure

An insertion mechanism for a drug delivery device. The insertion mechanism includes a proximal end, a distal end, a first opening disposed near the proximal end, and a second opening disposed in the distal end. A needle or cannula assembly is disposed within the housing and has a base with a proximal surface and a distal surface and a needle or cannula coupled to the distal surface. A retraction member is disposed within the housing to maintain the needle or cannula assembly in the retracted position before movement to the extended position, the retraction member in contact with the base. Upon supplying pressure through the first opening until an amount of pressure P1 applies an application force to the proximal surface of the base that surpasses a resistive force of the retraction member, the needle assembly is moved from the retracted position to the extended position.