An apparatus for aerosol medication delivery, comprising: a holding chamber having a first and second ends, the first end being adapted to accept a source of aerosol medicament; and a mouthpiece member provided at the second end of the holding chamber, for delivering aerosol medicament to a subject, the mouthpiece member comprising: a housing defining a passage through which aerosol medicament can be supplied to a subject and comprising a sidewall in which an opening is formed that opens to the outside of the housing; and a one-piece valve member comprising a first valve element in fluid communication with the mouthpiece and the holding chamber, the first valve element within the passage to permit delivery of aerosol to a subject during inhalation and blocks the passage during exhalation and a second valve clement in fluid communication with the mouthpiece and the opening in the sidewall, the second valve element to prevent air from reaching the subject through the opening in the sidewall of the housing during inhalation and permits the flow of air to the outside of the housing from the passage during exhalation, wherein the first valve element is a duckbill valve, characterized by: the housing defining the passage comprises a delivery member and an adaptor member, the adaptor member is provided at the second end of the holding chamber, and the delivery member is releasably connected to the adaptor member, wherein the opening in the sidewall is defined by a notch defined in an edge of the adaptor member and a notch defined in an edge of the delivery member. a holding chamber having a first and second ends, the first end being adapted to accept a source of aerosol medicament; and a mouthpiece member provided at the second end of the holding chamber, for delivering aerosol medicament to a subject, the mouthpiece member comprising: a housing defining a passage through which aerosol medicament can be supplied to a subject and comprising a sidewall in which an opening is formed that opens to the outside of the housing; and a one-piece valve member comprising a first valve element in fluid communication with the mouthpiece and the holding chamber, the first valve element within the passage to permit delivery of aerosol to a subject during inhalation and blocks the passage during exhalation and a second valve clement in fluid communication with the mouthpiece and the opening in the sidewall, the second valve element to prevent air from reaching the subject through the opening in the sidewall of the housing during inhalation and permits the flow of air to the outside of the housing from the passage during exhalation, wherein the first valve element is a duckbill valve, a housing defining a passage through which aerosol medicament can be supplied to a subject and comprising a sidewall in which an opening is formed that opens to the outside of the housing; and a one-piece valve member comprising a first valve element in fluid communication with the mouthpiece and the holding chamber, the first valve element within the passage to permit delivery of aerosol to a subject during inhalation and blocks the passage during exhalation and a second valve clement in fluid communication with the mouthpiece and the opening in the sidewall, the second valve element to prevent air from reaching the subject through the opening in the sidewall of the housing during inhalation and permits the flow of air to the outside of the housing from the passage during exhalation, wherein the first valve element is a duckbill valve, the housing defining the passage comprises a delivery member and an adaptor member, the adaptor member is provided at the second end of the holding chamber, and the delivery member is releasably connected to the adaptor member, wherein the opening in the sidewall is defined by a notch defined in an edge of the adaptor member and a notch defined in an edge of the delivery member.