A metered-dose inhaler (91; 131; 151), comprising: at least one vessel (101, 102; 167), said at least one vessel (101, 102; 167) including a first reservoir containing a first formulation and a second reservoir different from said first reservoir and containing a second formulation, and an actuator (92; 132-138; 152-158) for receiving said at least one vessel (101, 102), said metered-dose inhaler (91; 131; 151) being configured to be actuable when said at least one vessel (101, 102; 167) is received by said actuator (92; 132-138; 152-158), and being configured to simultaneously deliver at least a first metered dose of said first formulation from said first reservoir and a second metered dose of said second formulation from said second reservoir when said metered-dose inhaler (91; 131; 151) is actuated, wherein said first formulation is a first pressurized aerosol formulation and said second formulation is a second pressurized aerosol formulation, characterized in that said actuator comprises a guide member (138; 158) which defines a passageway for the first metered dose and another passageway for the second metered dose, an exit orifice of the guide member (138; 158) being in communication with an actuator block (134; 154) having a single nozzle orifice for atomizing both said first metered dose and said second metered dose upon actuation of said metered-dose inhaler (91; 131; 151); said second formulation comprises a low volatility component and is selected such that a mass median aerodynamic diameter, MMAD, of said first formulation is modulated by the presence of the low-volatility component in the second formulation when mixing said first metered dose and said second metered dose at an interconnection point of the first passageway and the second passageway, and prior to atomization at the nozzle orifice, causing a shift of the MMAD, which would be obtained by individually atomizing the first formulation, towards an MMAD which would be obtained by individually atomizing the second formulation. at least one vessel (101, 102; 167), said at least one vessel (101, 102; 167) including a first reservoir containing a first formulation and a second reservoir different from said first reservoir and containing a second formulation, and an actuator (92; 132-138; 152-158) for receiving said at least one vessel (101, 102), said metered-dose inhaler (91; 131; 151) being configured to be actuable when said at least one vessel (101, 102; 167) is received by said actuator (92; 132-138; 152-158), and being configured to simultaneously deliver at least a first metered dose of said first formulation from said first reservoir and a second metered dose of said second formulation from said second reservoir when said metered-dose inhaler (91; 131; 151) is actuated, wherein said first formulation is a first pressurized aerosol formulation and said second formulation is a second pressurized aerosol formulation, characterized in that said actuator comprises a guide member (138; 158) which defines a passageway for the first metered dose and another passageway for the second metered dose, an exit orifice of the guide member (138; 158) being in communication with an actuator block (134; 154) having a single nozzle orifice for atomizing both said first metered dose and said second metered dose upon actuation of said metered-dose inhaler (91; 131; 151); said second formulation comprises a low volatility component and is selected such that a mass median aerodynamic diameter, MMAD, of said first formulation is modulated by the presence of the low-volatility component in the second formulation when mixing said first metered dose and said second metered dose at an interconnection point of the first passageway and the second passageway, and prior to atomization at the nozzle orifice, causing a shift of the MMAD, which would be obtained by individually atomizing the first formulation, towards an MMAD which would be obtained by individually atomizing the second formulation.