doxazosin

Action and Spectrum

Action: 

Doxazosin is a selective alpha-1 adrenergic receptor antagonist that works by blocking alpha-1 receptors located on vascular smooth muscle and in the lower urinary tract. This leads to vasodilation, which reduces systemic vascular resistance and blood pressure, and causes relaxation of smooth muscle in the bladder neck and prostate, improving urinary flow and reducing symptoms of benign prostatic hyperplasia (BPH). 

Spectrum: 

Doxazosin is used primarily in the treatment of hypertension and BPH. In hypertension, it serves as a second-line or adjunctive agent, particularly when patients cannot tolerate other antihypertensive classes. In BPH, it effectively relieves lower urinary tract symptoms such as hesitancy, weak stream, and incomplete bladder emptying. 

Drug Interaction

Adverse Reaction

Adverse drug reactions:

Frequency defined 

>10% 

Headache 

Dizziness  

Fatigue  

1-10% 

Edema  

Rhinitis 

Dyspnea  

Nausea 

Abdominal pain  

Vertigo  

Upper respiratory tract infection (URTI) 

Hypotension  

Black Box Warning

None

Contraindication / Caution

Contraindication: 

Known hypersensitivity 

Caution: 

Liver disease 

Orthostatic hypotension 

Allergic reactions 

Pregnancy / Lactation

Pregnancy warnings:  

US FDA pregnancy category: Not Assigned

Breastfeeding warnings:

The release of the drug into the human breastmilk is known

Pregnancy Categories:    

Category A: Satisfactory and well-controlled studies show no risk to the fetus in the first or later trimester.

Category B: No evidence shown of risk to the fetus found in animal reproduction studies, and there are not enough studies on pregnant women

Category C: Adverse effects on the fetus found with evidence in animal reproduction studies and no adequate evidence for a result in humans must take care of potential risks in pregnant women

Category D: There is adequate data available with sufficient evidence of human fetal risk from various platforms, but despite the potential risk, and used only in emergency cases for potential benefits

Category X: Drugs listed in this category outweigh risks over benefits. Hence these categories of drugs need to be avoided by pregnant women.

Category N: There is no data available for the drug under this category

Pharmacology

Doxazosin is a selective alpha-1 adrenergic receptor antagonist that works by blocking alpha-1 receptors in vascular smooth muscle and the lower urinary tract. This blockade leads to vasodilation, resulting in decreased peripheral vascular resistance and lower blood pressure. In the bladder neck and prostate, doxazosin reduces smooth muscle tone, which helps improve urine flow and relieve symptoms in benign prostatic hyperplasia (BPH). 

Pharmacokinetics: 

Absorption 

Doxazosin is well absorbed after oral administration. The bioavailability is approximately 65% for the immediate-release formulation and 54-59% for the extended-release formulation. The peak antihypertensive effect occurs between 4 to 8 hours after dosing. 

Distribution 

Doxazosin is highly protein bound at approximately 99% and has a volume of distribution (Vd) ranging from 1 to 3.4 L/kg, indicating wide distribution in body tissues. 

Metabolism 

The drug undergoes extensive hepatic metabolism, producing several metabolites, including 6- and 7-O-demethyl derivatives, 6′- and 7′-hydroxy metabolites, and other minor products. 

Excretion and Elimination 

The elimination half-life is about 22 hours for immediate-release and 15 to 19 hours for extended-release formulations. Doxazosin is not dialyzable by hemodialysis. Its total body clearance ranges from 83 to 140 mL/min. The drug is primarily excreted in the feces (65%), with a smaller portion excreted via the urine (0.6-9%). 

Adminstartion

Doxazosin is administered orally and is available in both immediate-release and extended-release tablet formulations. 

Patient Information Leaflet

Patient information leaflet 

Generic Name: doxazosin 

Pronounced: Doks-AY-zoe-sin 

Why do we use doxazosin? 

Doxazosin is primarily used for the treatment of hypertension, where it lowers blood pressure by causing vasodilation through selective alpha-1 blockade. It is also widely used to manage benign prostatic hyperplasia (BPH), as it relaxes smooth muscle in the bladder neck and prostate, thereby improving urinary symptoms such as difficulty in starting urination, weak stream, and incomplete emptying. Additionally, doxazosin may be used off-label for conditions such as Raynaud’s phenomenon, ureteral stones (to aid in stone passage), and post-traumatic stress disorder (PTSD)-related nightmares, although these are not primary indications.