Apparatus for use in a drug mixing system including a vial adapter assembly including a main body element having a vial receiving portion and a needle puncturable port, the main body element including an axial hollow tubular portion which is in fluid flow engagement with a bore of a vial puncturing spike, the main body element further including a membrane support surface that supports a first membrane which is in fluid flow engagement with the vial puncturing spike via the bore and via a recess formed in an intermediate portion of the main body element, and a second membrane supported by a membrane support member and separated by a gap from the first membrane.
