Inhaler having in an axial direction a proximal end for insertion into the mouth of a user, and a distal end opposite to the proximal end, wherein the inhaler comprises an inlet, an outlet arranged at the proximal end, an air passage (10, 11, 12, 13) extending from the inlet to the outlet, and a reservoir (8, 9) communicating with the air passage (10, 11, 12, 13) through a release orifice (6, 7), the reservoir (8, 9) containing a dispersible substance, wherein the inhaler has a proximal part comprising the outlet, and a distal part attached to the proximal part, wherein the proximal part is linearly slidable along the axial direction with respect to the distal part between an 'OPEN' position where the proximal part is deployed from the distal part, and a 'CLOSED' position where the proximal part is retracted towards the distal part, wherein the inhaler further comprises an inlet valve member, and an outlet valve member; - wherein the reservoir (8, 9) is arranged in the proximal part; - wherein the inlet comprises one or more openings in a peripheral housing wall of the proximal part, the one or more openings facing radially outward, away from the axial direction, and the inlet valve member is formed by a peripheral housing wall of the distal part covering the openings when the proximal part is in the 'CLOSED' position; - wherein the outlet comprises an axially oriented aperture, wherein the outlet valve member is formed as a plug attached to the distal part, the plug blocking the outlet when the proximal part is in the 'CLOSED' position; - wherein the release orifice (6, 7) is oriented in the axial direction; and - wherein the inhaler further comprises a reservoir valve member (16, 17), wherein the reservoir valve member (16, 17) is formed as a peg travelling in the axial direction, the peg being fixed to the distal part via an axially extending stem, wherein the peg blocks the release orifice (6, 7) when the proximal part is in the 'CLOSED' position; whereby said inlet valve member, outlet valve member and reservoir valve member (16, 17) are arranged so as - to simultaneously close the inlet, the outlet, and the release orifice (6, 7) when the proximal part is retracted towards the distal part along the axial direction from the 'OPEN' position into the 'CLOSED' position, and - to simultaneously open the inlet, open the outlet, and deliver an amount of the dispersible substance from the reservoir (8, 9) through the release orifice (6, 7) to the air passage (10, 11, 12, 13) when the proximal part is deployed from the distal part along the axial direction from the 'CLOSED' position into the 'OPEN' position. - wherein the reservoir (8, 9) is arranged in the proximal part; - wherein the inlet comprises one or more openings in a peripheral housing wall of the proximal part, the one or more openings facing radially outward, away from the axial direction, and the inlet valve member is formed by a peripheral housing wall of the distal part covering the openings when the proximal part is in the 'CLOSED' position; - wherein the outlet comprises an axially oriented aperture, - wherein the release orifice (6, 7) is oriented in the axial direction; and - wherein the inhaler further comprises a reservoir valve member (16, 17), wherein the reservoir valve member (16, 17) is formed as a peg travelling in the axial direction, the peg being fixed to the distal part via an axially extending stem, wherein the peg blocks the release orifice (6, 7) when the proximal part is in the 'CLOSED' position; - to simultaneously close the inlet, the outlet, and the release orifice (6, 7) when the proximal part is retracted towards the distal part along the axial direction from the 'OPEN' position into the 'CLOSED' position, and - to simultaneously open the inlet, open the outlet, and deliver an amount of the dispersible substance from the reservoir (8, 9) through the release orifice (6, 7) to the air passage (10, 11, 12, 13) when the proximal part is deployed from the distal part along the axial direction from the 'CLOSED' position into the 'OPEN' position.