Abstract: A dry powder inhaler comprises a disc-shaped assembly (14) having cavities (16) filled with medicament powder for inhalation. The assembly is rotatably mounted in a housing (6, 8) on which is mounted a rotatable external casing (11, 12). A mouthpiece (10) is movably mounted on the housing (6, 8) allowing it to be brought into and out of engagement with the edge of the disc (14) whereby the mouthpiece acts as a brake to prevent or permit rotational movement of the disc (14). The inhaler is opened by rotating the housing with respect to the casing (11, 12) to expose the mouthpiece (10). As the inhaler is opened and closed, cam followers (105) on the mouthpiece move along cam tracks (109) in the casing (11, 12), causing the mouthpiece to move against or away from the disc (14). A gap is defined between the disc and mouthpiece to allow a flow of bypass air to enter the mouthpiece.