Alpha-1 (ɑ1) Blockers

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 in the treatment of hypertension. It is to be noted, they are not used as first-line agents for hypertension treatment.

Created in BioRender. Boucher, M. (2025) https://BioRender.com/234s2nk

Fig 2: Mechanism of action of alpha-1 adrenergic receptor blockers

Pharmacokinetic Parameters

Although all α1-adrenoceptor blockers share a similar mechanism of action, their pharmacokinetic profiles differ in ways that influence dosing schedules, onset of action, duration of effect, and potential for drug interactions. The table below summarises and compares the principal pharmacokinetic characteristics of tamsulosin, alfuzosin, terazosin and doxazosin, highlighting clinically relevant distinctions between these agents.

Contraindications

Alpha blockers are generally well tolerated, but several important contraindications must be considered before initiating therapy. These agents should be avoided in patients with a history of postural hypotension or micturition syncope, due to the increased risk of symptomatic blood pressure drops and falls.

Significant hepatic impairment is also a key concern: tamsulosin and terazosin are contraindicated in severe hepatic insufficiency, while immediate-release alfuzosin is contraindicated in severe hepatic failure. Modified-release alfuzosin should not be used in patients with hepatic failure or in those with a creatinine clearance (CrCl) below 30 mL/min.

Additionally, alfuzosin should not be co-administered with other α1-blockers or dopamine agonists because of the risk of additive hypotensive effects. Furthermore, doxazosin is contraindicated in patients with benign prostatic hyperplasia (BPH) who also have upper urinary tract congestion, chronic urinary tract infection, or bladder stones, as these underlying conditions require alternative management strategies.

Cautions

Caution is advised when prescribing α1-adrenoceptor blockers to the elderly, as they may cause hypotension and, rarely, syncope – particularly after the first dose. Furthermore, concomitant use with other antihypertensive agents may increase the risk of additive blood pressure lowering effects. Renal impairment also requires consideration; tamsulosin should be used cautiously in patients with a creatinine clearance (CrCl) below 10 mL/min. Furthermore, angioedema has been reported with tamsulosin, and treatment should not be restarted if this occurs.

All α1-adrenoceptor blockers have been associated with Intraoperative Floppy Iris Syndrome (IFIS), although the risk appears higher with tamsulosin, and ophthalmologists should be informed prior to cataract surgery.

Alfuzosin should be used cautiously in patients with a history of QT prolongation and in those with mild to moderate hepatic impairment (immediate-release formulation), while doxazosin requires caution in mild to moderate hepatic impairment and should be avoided in severe hepatic impairment.

Additionally, alpha blockers should be used carefully in patients with certain acute cardiac conditions, including pulmonary oedema due to aortic or mitral stenosis, high-output cardiac failure, right-sided heart failure due to pulmonary embolism or pericardial effusion, and left ventricular heart failure with low filling pressure.

Interactions

CYP3A4 mediated interactions:

Tamsulosin: Strong CYP3A4 inhibitors should be avoided in patients who are CYP2D6 poor metabolisers. Caution is advised when tamsulosin is used concomitantly with strong or moderate CYP3A4 inhibitors.

Alfuzosin: Concomitant use with strong CYP3A4 inhibitors is contraindicated.

Doxazosin: Use with strong CYP3A4 inhibitors should be approached with caution.

Table 1: 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 Effects

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

1. https://bnf.nice.org.uk/treatment-summaries/lower-urinary-tract-symptoms-in-males/
2. https://www.medicines.org.uk/emc/product/9245/smpc#gref
3. https://bnf.nice.org.uk/drugs/tamsulosin-hydrochloride/
4. https://bnf.nice.org.uk/drugs/alfuzosin-hydrochloride/
5. https://www.medicines.org.uk/emc/product/5205/smpc#gref
6. https://www.medicines.org.uk/emc/product/1182/smpc#gref
7. https://bnf.nice.org.uk/drugs/terazosin/#medicinal-forms
8. https://bnf.nice.org.uk/drugs/doxazosin/#indications-and-dose
9. https://www.medicines.org.uk/emc/product/4075/smpc#gref