A drug delivery system comprising: a drug delivery pen having a generally tubular pen housing that extends from a first end to a second end, the first end of the housing enclosing at least a portion of a plunger rod coupled to a drug cartridge disposed proximate the second end of the housing, the first end of the pen housing having a dosage selector coupled to the plunger rod, the drug delivery pen adapted such that rotation of the dosage selector causes it to telescope in a linear direction; and an add-on module housing extending along a first longitudinal axis from a first housing end to a second housing end, the add-on module housing including first and second extensions that circumscribe about a second longitudinal axis to define at least a portion of a hollow bore and partially circumscribe a portion of the first end of the pen housing for attachment of the add-on module housing to the drug delivery pen, the second longitudinal axis extending along a center point of the hollow bore and the first and second longitudinal axes (L1, L2) being generally parallel, the add-on module housing further comprising: a dosage sensor coupled to the add-on module housing a follower portion directly connected to the dosage sensor and configured to be engaged by the dosage selector such that the follower portion follows the axial movement of the dosage selector along its axis as dosage is adjusted and the dosage sensor is configured to measure the position of the dosage selector for determining the size of the bolus injected by a user, in which the dosage sensor includes a longitudinal member movable along the longitudinal axis, the longitudinal member connected to the follower portion that extends from the housing proximate the second housing end, in which the longitudinal member comprises a slider disposed on a potentiometer, the slider being slidable on the potentiometer along the longitudinal axis between proximate the first housing end and the second housing end upon movement of a knob, wherein the knob is coupled to the add-on module housing and comprises a through-bore in alignment with hollow bore and the knob is constrained to move with follower portion as knob is moved axially by rotating knob about the second longitudinal axis. a drug delivery pen having a generally tubular pen housing that extends from a first end to a second end, the first end of the housing enclosing at least a portion of a plunger rod coupled to a drug cartridge disposed proximate the second end of the housing, the first end of the pen housing having a dosage selector coupled to the plunger rod, the drug delivery pen adapted such that rotation of the dosage selector causes it to telescope in a linear direction; and an add-on module housing extending along a first longitudinal axis from a first housing end to a second housing end, the add-on module housing including first and second extensions that circumscribe about a second longitudinal axis to define at least a portion of a hollow bore and partially circumscribe a portion of the first end of the pen housing for attachment of the add-on module housing to the drug delivery pen, the second longitudinal axis extending along a center point of the hollow bore and the first and second longitudinal axes (L1, L2) being generally parallel, the add-on module housing further comprising: a dosage sensor coupled to the add-on module housing a follower portion directly connected to the dosage sensor and configured to be engaged by the dosage selector such that the follower portion follows the axial movement of the dosage selector along its axis as dosage is adjusted and the dosage sensor is configured to measure the position of the dosage selector for determining the size of the bolus injected by a user, a dosage sensor coupled to the add-on module housing a follower portion directly connected to the dosage sensor and configured to be engaged by the dosage selector such that the follower portion follows the axial movement of the dosage selector along its axis as dosage is adjusted and the dosage sensor is configured to measure the position of the dosage selector for determining the size of the bolus injected by a user, in which the dosage sensor includes a longitudinal member movable along the longitudinal axis, the longitudinal member connected to the follower portion that extends from the housing proximate the second housing end, in which the longitudinal member comprises a slider disposed on a potentiometer, the slider being slidable on the potentiometer along the longitudinal axis between proximate the first housing end and the second housing end upon movement of a knob, wherein the knob is coupled to the add-on module housing and comprises a through-bore in alignment with hollow bore and the knob is constrained to move with follower portion as knob is moved axially by rotating knob about the second longitudinal axis.