ɑ1 blockers used for urinary retention associated with benign prostatic hyperplasia include alfuzosin hydrochloride, tamsulosin, doxazosin and terazosin. Doxazosin and terazosin can also be used to treat hypertension. BPH can lead to urinary retention, and is associated with intravesicle obstruction triggered by anatomical and functional factors. Tension of prostatic smooth muscle, mediated by ɑ1 receptors, causes the functional component of obstruction. ɑ1 blockers provide rapid relief for both voiding and storage urinary symptoms; however, they do not alter the progression of BPH. Adobe stockFig 1: Benign prostatic hyperplasia: normal vs. enlarged prostate gland Mechanism Of Action Tamsulosin, alfuzosin, doxazosin and terazosin are all antagonists at postsynaptic ɑ1-adrenoceptors. Blockade of ɑ1-adrenoceptors in the prostate, bladder, and prostatic urethra leads to relaxation of smooth muscle, improving urinary flow by decreasing obstruction caused by an enlarged prostate. Doxazosin and terazosin additionally block ɑ1-adrenoceptors in vascular smooth muscle, leading to vasodilation and hypotension, making them useful for hypertension. Created in BioRender. Boucher, M. (2025) https://BioRender.com/234s2nk Fig 2: Mechanism of action of alpha-1 adrenergic receptor blockers Pharmacokinetic Parameters Drug Formulation Absorption / Tmax Distribution Metabolism Excretion / Half-life Tamsulosin Prolonged-release capsule F ≈ 100%, Tmax ≈ 6 h, Css by day 5 Plasma protein binding 99%, Vd 0.2 L/kg Not extensively metabolised CYP3A4, CYP2D6 Mainly urine; t½ 10 h (single dose), 13 h (steady state); no renal/hepatic dose adjustment required Alfuzosin Immediate & modified release tablet F = 64%, Tmax 0.5–6 h (immediate release) Plasma protein binding 90% CYP3A4 Urine/faeces; t½ 3–5 h Terazosin Standard-release tablet F 80–100%, Tmax 1 h Plasma protein binding 90–94% Demethylation & conjugation t½ 12 h; 40% urine, 60% feces Doxazosin Standard & modified-release tablet F ≈ 66% (standard release) Plasma protein binding 98% CYP3A4 mainly, minor CYP2D6 & CYP2C9 t½ 22 h; no PK change in renal impairment Cautions And Contraindications The following cautions and contraindications should be noted for ɑ1-adrenoceptor blockers: Cautions Contraindications Hypotension and rarely syncope, especially after the first dose (higher risk in elderly) CrCl <10ml/min (tamsulosin) Angioedema – tamsulosin should not be restarted if occurs Intraoperative Floppy Iris Syndrome (risk of all ɑ1-adrenoceptor blockers but higher with tamsulosin) Concomitant antihypertensives History of QT prolongation (alfuzosin) Mild to moderate hepatic failure (alfuzosin immediate-release) Mild to moderate hepatic impairment, avoid in severe hepatic impairment (doxazosin) Acute cardiac conditions such as: pulmonary oedema due to aortic or mitral stenosis high-output cardiac failure right-sided heart failure due to pulmonary embolism or pericardial effusion Left ventricular heart failure with low filling pressure. History of postural hypotension or micturition syncope Severe hepatic insufficiency (tamsulosin, terazosin) Severe hepatic failure (Immediate release alfuzosin) Hepatic failure and crcl <30ml/min (Modified-release alfuzosin) Alfuzosin with other ɑ1 blockers or dopamine agonists Patients with BPH and concomitant congestion of the upper urinary tract, chronic urinary tract infection or bladder stones (doxazosin) Interactions CYP3A4 mediated interactions: Tamsulosin: Avoid strong inhibitors of CYP3A4 in patients with poor metaboliser CYP2D6 phenotype. Use caution in combination with strong and moderate inhibitors of CYP3A4. Alfuzosin: not to be given with potent CYP3A4 inhibitors Doxazosin: caution with potent CYP3A4 inhibitors Created with ChatGTP Fig 3: CYP3A4 inducers and inhibitors Other Interactions: Concomitant alpha1-blockers or antihypertensives may increase risk of hypotension. Administration of terazosin or doxazosin (immediate-release) should be avoided for 4 and 6 hours respectively after phosphodiesterase-5-inhibitors (e.g. sildenafil, tadalafil, vardenafil) as the hypotensive effect is enhanced Adverse Events Prolonged erections and priapism have been reported with ɑ1-adrenoceptor blockers. Furthermore, tamsulosin can commonly cause ejaculation disorder, retrograde ejaculation and ejaculation failure. All ɑ1-adrenoceptor blockers can cause postural hypotension and dizziness. Vasodilatory effects are more pronounced with doxazosin and terazosin than with alfuzosin and tamsulosin. Other described side effects include Alfuzosin commonly causes nausea, diarrhoea, dry mouth and vertigo Terazosin side effects include: angioedema, gastrointestinal discomfort, sexual dysfunction and thrombocytopenia (frequency unknown) Doxazosin commonly increases risk of infection. Uncommonly doxazosin can cause sexual dysfunction and very rarely gynaecomastia. References https://bnf.nice.org.uk/treatment-summaries/lower-urinary-tract-symptoms-in-males/ https://www.medicines.org.uk/emc/product/9245/smpc#gref https://bnf.nice.org.uk/drugs/tamsulosin-hydrochloride/ https://bnf.nice.org.uk/drugs/alfuzosin-hydrochloride/ https://www.medicines.org.uk/emc/product/5205/smpc#gref https://www.medicines.org.uk/emc/product/1182/smpc#gref https://bnf.nice.org.uk/drugs/terazosin/#medicinal-forms https://bnf.nice.org.uk/drugs/doxazosin/#indications-and-dose https://www.medicines.org.uk/emc/product/4075/smpc#gref Do you think you’re ready? Take the quiz below Pro Feature - Quiz Alpha-1 (ɑ1) Blockers Question 1 of 3 Submitting... Skip Next Rate question: You scored 0% Skipped: 0/3 More Questions Available Upgrade to TeachMePharmacy Pro Challenge yourself with over 2100 multiple-choice questions to reinforce learning Learn More Rate This Article