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Sildosin mode of action

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silodosin

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silodosin mechanism of action selective alpha-1A adrenergic receptor FDA label

Silodosin is a highly selective alpha-1A adrenergic receptor blocker.

Mechanism of action

  • Alpha-1A receptors are concentrated in the prostate, prostatic urethra, and bladder neck.
  • Silodosin blocks norepinephrine-mediated stimulation of these receptors.
  • This relaxes smooth muscle in the prostate and bladder outlet, reducing urethral resistance and improving urine flow and lower urinary tract symptoms in benign prostatic hyperplasia (BPH).
  • It does not shrink the prostate or lower PSA significantly. It improves the dynamic, smooth-muscle component of obstruction.
It has much greater affinity for alpha-1A than alpha-1B receptors, so it is relatively uroselective and generally causes less vascular alpha-blockade than nonselective alpha-blockers. Its high alpha-1A selectivity also explains the common adverse effect of reduced or absent semen emission/ejaculation, which is usually reversible after stopping it. Campbell-Walsh-Wein Urology, section “Silodosin.”
Because it can still lower blood pressure, dizziness or postural hypotension may occur, particularly with antihypertensives or PDE-5 inhibitors such as sildenafil/tadalafil.

Why it causes postural hypotension

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silodosin orthostatic hypotension

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silodosin prescribing information orthostatic hypotension alpha adrenergic receptor

On standing, blood briefly pools in the veins of the legs and abdomen. Normally, sympathetic norepinephrine activates vascular alpha-1 receptors, causing arteriolar and venous constriction to maintain blood pressure and cerebral blood flow.
Silodosin mainly blocks alpha-1A receptors in the prostate, but it is not perfectly confined there. Some blockade of vascular alpha-1 receptors reduces this compensatory vasoconstriction.
Therefore, after standing:
  1. Venous pooling lowers venous return to the heart.
  2. Cardiac output transiently falls.
  3. Blood vessels cannot constrict adequately because of alpha-blockade.
  4. Blood pressure falls, producing dizziness, light-headedness, blurred vision, or rarely syncope.
This is more likely when starting treatment, in older adults, dehydration, renal impairment, or with other blood-pressure-lowering drugs, alcohol, or PDE-5 inhibitors. Silodosin is relatively alpha-1A selective, so postural hypotension is less prominent than with less selective alpha-blockers, but it can still occur. The DailyMed prescribing information reports orthostatic hypotension in 2.6% of silodosin-treated participants versus 1.5% with placebo in 12-week trials.

Prostagardlde

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Prostagarde

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"Prostagarde" medicine

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Prostagarde

Prostagard is a brand name for silodosin (usually 4 mg or 8 mg capsules).
It is an alpha-1A blocker used to relieve urinary symptoms of benign prostatic hyperplasia (BPH), such as weak urine stream, straining, frequent urination, and nocturia.
So, the mechanism and postural hypotension explanation above apply to Prostagard as well:
  • It relaxes smooth muscle in the prostate and bladder neck.
  • Some vascular alpha-receptor blockade can impair the normal vasoconstrictor response on standing.
  • Blood pressure may temporarily fall, causing dizziness or faintness.
Take it with food as prescribed, rise slowly from sitting or lying down, and seek medical advice if fainting occurs. It commonly causes reduced or absent ejaculation, dizziness, nasal congestion, and sometimes diarrhea. A Prostagard product listing identifies its active ingredient as silodosin.

Sildoease

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"Sildoease" capsule medicine composition

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Sildoease

Sildoease is a brand of silodosin, usually 8 mg.
It is used for urinary symptoms caused by benign prostatic hyperplasia (BPH), an enlarged prostate. It relaxes the prostate and bladder-neck muscles by blocking alpha-1A receptors, improving urinary flow. It does not reduce prostate size.
Common side effects:
  • Dizziness, especially on standing
  • Postural hypotension
  • Reduced or absent ejaculation
  • Nasal congestion
  • Headache or diarrhea
There is also Sildoease 4 D, which contains silodosin 4 mg + dutasteride 0.5 mg. Dutasteride gradually shrinks the prostate, unlike silodosin. Check your strip carefully because the two products have different ingredients. A Sildoease product listing identifies Sildoease 8 mg as silodosin.

Dutasteride

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dutasteride benign prostatic hyperplasia

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dutasteride prescribing information mechanism warnings pregnancy PSA DailyMed

Dutasteride is a 5-alpha-reductase inhibitor used for benign prostatic hyperplasia (BPH).

Mechanism of action

It inhibits both type 1 and type 2 forms of 5-alpha-reductase, the enzyme that converts testosterone into dihydrotestosterone (DHT).
↓ DHT leads to:
  • Gradual shrinkage of the enlarged prostate
  • Reduced obstruction to urine flow
  • Lower risk of acute urinary retention and BPH-related surgery
It works slowly, often requiring 3 to 6 months for substantial benefit. In contrast, silodosin relaxes prostate muscle and can improve urine flow sooner.

In Sildoease 4 D

  • Silodosin 4 mg: quick relief by relaxing prostate/bladder-neck smooth muscle
  • Dutasteride 0.5 mg: long-term reduction in prostate size

Important effects and cautions

  • May reduce libido, erectile function, and semen volume.
  • It lowers PSA by approximately 50%, so the clinician must interpret PSA results accordingly during follow-up.
  • Women who are pregnant or might become pregnant should not handle leaking capsules, because skin absorption could harm development of a male fetus.
  • It is not a medicine for immediate relief of urinary retention.
The DailyMed dutasteride label describes its DHT-lowering action and PSA/pregnancy precautions.
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