Alpha-Blockers in Allergic Rhinitis (MAN 01)

Inclusion Criteria: * Male or female 18-70 years * Able to provide informed consent * Able to comply with the requirements of the protocol * Diagnosis of allergic rhinitis * Peak nasal inspiratory flow which is reversible to \>20 l/min with oxymetazoline * Positive skin prick test to at least one common aeroallergen * Ability to withhold antihistamines, intranasal corticosteroids for duration of the study * Agreement for their GP to be made aware of study participation and to receive feedback as relevant to the participant's well being Exclusion Criteria: * Pregnancy, planned pregnancy or lactation * Respiratory tract infection in the previous 2 months * Nasal polyps of more than Grade I on nasal endoscopy * Any known adverse reaction to the Investigational Medicinal Product or other quinazolines * Systolic BP \<100mmHg * Any degree of heart block * Any clinically significant medical condition that may endanger the health or safety of the participant or jeopardise the protocol * Participation within another clinical trial of investigational medicinal product within the last 30 days * Under the age of 18 * Benign hyperplasia, chronic urinary tract infections, bladder stones, overflow bladder, anuria or progressive renal failure * History of oesophageal or gastrointestinal obstruction or decreased lumen diameter of the gastrointestinal tract. * Concomitant use of PDE5 inhibitors (sildenafil etc.) * Alpha antagonists - this is the medication under investigation. * Other cardiovascular medications including but not limited to: ACE inhibitors, Angiotensin II antagonists, Beta blockers, calcium channel blockers, diuretics, nitrates, phosphodiesterase type-5 inhibitors or other vasodilating medications - these in combination with doxazosin carry an increased risk of symptomatic hypotension. * Monoamine oxidase inhibitors (MAOIs) - antidepressant medication that can lead to significant hypotension in combination with alpha antagonists. * Anxiolytics and hypnotics - enhanced hypotensive and sedative effects when alpha-blockers given in combination with these. * Methyldopa and levodopa - potential enhanced hypotensive effect with alpha antagonists. * Nasal corticosteroid spray - would confound study outcomes * Oxymetazoline - this is used as an outcome of response in the study. * Antihistamines - the participants will be undergoing a histamine nasal challenge. * Sympathomimetics i.e. dopamine, ephedrine, epinephrine, metaraminol, methoxamine, phenylephrine- may reduce the effect of doxazosin. * Any other medication, which in the opinion of the Investigator may put the participant at risk.