An inhaler device comprising a container for powdered medicament, an inhalation channel, a movable metering member, a depressible actuator and a multiple dosage preventing means for locking the depressible actuator after use such that the actuator is unlocked only upon inhalation. The inhaler device is useful in the treatment of respiratory diseases such as asthma. The multiple dosage preventing means of the device eliminates the risk of unintentional overdosing.