Apparatus (300; 301; 401; 501) for generating an inhalable medium, the apparatus comprising: a first receptacle (302; 303; 403; 503) configured to receive a first substrate (304; 305; 405; 505), wherein the first substrate is a solid or a gel; a second receptacle (306; 307; 407; 507) configured to receive a second substrate (308; 309; 409; 509), wherein the second substrate is a solid or a gel; a first heater (310; 311; 411; 511) arranged to heat the first substrate in use; a second heater (312; 313; 413; 513) arranged to heat the second substrate in use; characterized by : a housing defining: a first flow path (350; 351; 451; 551) including the first receptacle and the second receptacle; and a second flow path (360; 361; 461; 561) including the first receptacle and not the second receptacle; wherein: the first flow path is arranged such that in use heated products produced by heating the first substrate received in the first receptacle pass through the second receptacle to produce the inhalable medium; and the second flow path is arranged such that in use heated products produced by the first substrate received in the first receptacle form the inhalable medium and pass along the second flow path without passing through the second receptacle. a first receptacle (302; 303; 403; 503) configured to receive a first substrate (304; 305; 405; 505), wherein the first substrate is a solid or a gel; a second receptacle (306; 307; 407; 507) configured to receive a second substrate (308; 309; 409; 509), wherein the second substrate is a solid or a gel; a first heater (310; 311; 411; 511) arranged to heat the first substrate in use; a second heater (312; 313; 413; 513) arranged to heat the second substrate in use; characterized by : a housing defining: a first flow path (350; 351; 451; 551) including the first receptacle and the second receptacle; and a second flow path (360; 361; 461; 561) including the first receptacle and not the second receptacle; wherein: the first flow path is arranged such that in use heated products produced by heating the first substrate received in the first receptacle pass through the second receptacle to produce the inhalable medium; and the second flow path is arranged such that in use heated products produced by the first substrate received in the first receptacle form the inhalable medium and pass along the second flow path without passing through the second receptacle. a housing defining: a first flow path (350; 351; 451; 551) including the first receptacle and the second receptacle; and a second flow path (360; 361; 461; 561) including the first receptacle and not the second receptacle; a first flow path (350; 351; 451; 551) including the first receptacle and the second receptacle; and a second flow path (360; 361; 461; 561) including the first receptacle and not the second receptacle; wherein: the first flow path is arranged such that in use heated products produced by heating the first substrate received in the first receptacle pass through the second receptacle to produce the inhalable medium; and the second flow path is arranged such that in use heated products produced by the first substrate received in the first receptacle form the inhalable medium and pass along the second flow path without passing through the second receptacle. the first flow path is arranged such that in use heated products produced by heating the first substrate received in the first receptacle pass through the second receptacle to produce the inhalable medium; and the second flow path is arranged such that in use heated products produced by the first substrate received in the first receptacle form the inhalable medium and pass along the second flow path without passing through the second receptacle. <EOS>