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Prazosin exerts a selective postsynaptic action, inhibiting the binding of norepinephrine to the α1 receptors.
Agents that are selective α1 antagonists include also terazosin, doxazosin, alfuzosin, silodosin and tamsulosin.

Three α1 receptor subtypes have been identified, α1a, α1b and α1d:

  • α1a and α1d subtypes are predominant in the prostate, bladder, and detrusor muscle.
  • α1b subtypes are largely located on vascular smooth muscle.

Prazosin antagonizes all three subtypes; blockade of α1a and α1d can lead to smooth muscle relaxation in the lower urinary tract resulting in improved urinary flow rates. At the same time blockade of α1b subtypes can cause relaxation in peripheral vascular resistance which results in a decrease in blood pressure.

Prazosin is not indicated for the treatment of the signs and symptoms of benign prostatic hyperplasia, as for this purpose there are more selective drugs for α1a and α1d receptor subtypes with less unwanted systemic effects (e.g. tamsulosin )

Prazosin is limited by its short half-life (2.5 h), which requires dosing 2 to 3 times daily


[edit] DOSAGE

Prazosin requires multiple daily dosing; the initial dosage in patients with hypertension is 1 mg two or three times a day. This starting dose is intended to minimize the frequency of postural hypotension and first-dose syncope. Dosage may then be slowly increased to a total daily dose of 20 mg given in divided doses. The therapeutic dosages most commonly employed have ranged from 6 mg to 15 mg daily given in divided doses


  • Hypersensitivity to Prazosin


  • Postural hypotension, dizziness, lightheadedness and syncope can occur, especially at initiation of therapy or at the time of dose increases (syncopal episodes have usually occurred within 30 to 90 minutes of the initial dose of the drug). Treatment should be initiated with the 1 mg dose and should then be adjusted slowly to the recommended dose.
  • Avoid driving or performing hazardous tasks for the first 24 hours after taking this medicine or when the dose is increased (Dizziness or drowsiness may occur after the first dose)


  • Concomitant administration of Prazosin with a PDE5 inhibitor (Viagra, Levitra, Cialis, Stendra) can result in additive blood pressure lowering effects and symptomatic hypotension.


  • Pregnancy Category C (US). There are no adequate and well controlled studies which establish the safety of Prazosin in pregnant women. Therefore, it should be used during pregnancy only if the potential benefit justifies the potential risk to the mother and fetus.
  • Nursing mothers: Prazosin has been shown to be excreted in small amounts in human milk, therefore, caution should be exercised if administered to a nursing mother.


The most commonly adverse reactions are asthenia, headache, postural hypotension, dizziness, drowsiness, palpitations, nausea and nasal congestion . Side effects have disappeared with continued therapy or have been tolerated with no decrease in dose of drug.


Benign Prostatic Hyperplasia (BPH) and Treatments (Video 1)
Benign Prostatic Hyperplasia (BPH) and Treatments (Video 2)



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