An insertion mechanism having a vented fluid pathway includes an insertion biasing member 210, a hub 212, a needle 214, a retraction biasing member 216, and a manifold 240 having a septum 230 and a cannula 234, wherein the annular space between the septum 230 and the cannula 234 defines a manifold header 242. The manifold and fluid conduit are ventable through the membrane prior to or just after needle 214 and cannula 234 are inserted into the user. Such insertion is caused by the insertion biasing member(s) 210, after which only the needle 214 or another occlusion element is retracted. Such retraction may open a fluid pathway from the manifold header 242 to the body through the cannula 234 and/or needle 214. A drug delivery pump 10 includes such an insertion mechanism 200 having a vented fluid pathway. The drug delivery pump 10 may contain the insertion mechanism 200 having a vented fluid pathway in either an internally i or an externally tethered configuration.