A method of delivering a substance, such as one or more of a triptan, a nasal steroid or carbon dioxide gas, to the nasal cavity of a subject, in particular for the treatment of headaches, for example, migraine, or rhinosinusitis, for example, chronic rhinosinusitis, optionally with polyps, the method comprising the steps of fitting a nosepiece to one nostril of the subject, delivering the substance through the nosepiece to the posterior region of the nasal cavity of the subject.