A nasal cannula comprising: a base portion defining a gas passageway and one or more nozzles defining a second gas passageway in gaseous communication with the first gas passageway for directing a therapeutic flow of gas to a user's nares. The nasal cannula may include one or more sensors for measuring the properties of gas within a user's nares. The nozzle may be a nasal insert that is inserted into the user's nares; recesses or grooves are provided for preventing sealing of the nasal insert with the nare. The nozzle may be shaped to avoid insertion into the user's nares, thus preventing sealing with the nares. A stop may be positioned between two nozzles to engage a user's columella and prevent the nozzles from inserting into the user's nares. Elongate extensions are provided for inserting into the user's nares and supporting sensors for measuring gas properties therein.