An auto-injector for delivering a liquid medicament comprising: a housing arranged to contain a syringe with a piston for sealing the syringe and displacing the medicament, the housing having a proximal end and a distal end intended to be applied against an injection site; and a telescopic plunger unit arranged between the piston and the proximal end of the housing, the telescopic plunger unit comprising: a telescopic frame comprising a plurality of telescopic plunger parts (192, 194, 196), and each of the plurality of telescopic plunger parts has a conical frustum shape with an increasing inner diameter towards one end of the housing, wherein the telescopic frame is made of a compressible material such that friction from contact between an inner surface of one of the plurality of telescopic plunger parts and an outer surface of another of the plurality of telescopic plunger parts is provided throughout the extension of the telescopic frame, and a spring element surrounding a plunger element, wherein the plunger element is fixed to a base of the telescopic plunger unit and the spring element is held in an initial compressed state by a compressive force exerted by the base of the telescopic plunger unit; wherein when the spring element is released, a tapering profile of the telescopic frame is configured to provide varying friction with extension such that resistance to telescopic extension decreases as spring force provided by the spring element decreases with increasing extension of the telescopic plunger unit, and wherein a combination of the decreasing spring force provided by the spring element and the decreasing resistance to telescopic extension provides a substantially constant resultant driving force exerted on the base of the telescopic plunger unit, such that a substantially constant pushing force is exerted on the piston when the spring element is released. a housing arranged to contain a syringe with a piston for sealing the syringe and displacing the medicament, the housing having a proximal end and a distal end intended to be applied against an injection site; and a telescopic plunger unit arranged between the piston and the proximal end of the housing, the telescopic plunger unit comprising: a telescopic frame comprising a plurality of telescopic plunger parts (192, 194, 196), and each of the plurality of telescopic plunger parts has a conical frustum shape with an increasing inner diameter towards one end of the housing, wherein the telescopic frame is made of a compressible material such that friction from contact between an inner surface of one of the plurality of telescopic plunger parts and an outer surface of another of the plurality of telescopic plunger parts is provided throughout the extension of the telescopic frame, and a spring element surrounding a plunger element, wherein the plunger element is fixed to a base of the telescopic plunger unit and the spring element is held in an initial compressed state by a compressive force exerted by the base of the telescopic plunger unit; wherein when the spring element is released, a tapering profile of the telescopic frame is configured to provide varying friction with extension such that resistance to telescopic extension decreases as spring force provided by the spring element decreases with increasing extension of the telescopic plunger unit, and wherein a combination of the decreasing spring force provided by the spring element and the decreasing resistance to telescopic extension provides a substantially constant resultant driving force exerted on the base of the telescopic plunger unit, such that a substantially constant pushing force is exerted on the piston when the spring element is released.