A monodose inhaler for powdered medicaments consists of a hollow substantially pipe-shaped body that has a first portion (H), for housing a capsule or cartridge of powdered medicament, defined by a wall (W) in which there are formed slots (S) as air intakes to an inner region where the powder drops, and a second portion (M) connected to the first portion (H) for delivering the medicament by means of a primary stream (F) that carries the powder from the drop region along a delivery duct (D) whose end is suitable to be placed in the patient's mouth, and it further includes a secondary duct (D′) located under the delivery duct (D) and provided with its own air intake (S′) for delivering a powderless secondary stream (F′).