Which drug for proper erection

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Here is a comprehensive overview of drugs used to treat erectile dysfunction (ED), based on authoritative pharmacology textbooks:

Drugs for Erectile Dysfunction

Mechanism of Normal Erection

Erection depends on nitric oxide (NO) release from penile nerves and endothelium. NO activates guanylyl cyclase → increases cGMP → smooth muscle relaxation → blood engorgement of corpus cavernosum → erection. PDE5 is the enzyme that breaks down cGMP; blocking it prolongs erection.

1. PDE5 Inhibitors (First-Line, Oral)

These are the mainstay of treatment for ED and work by inhibiting PDE5, preventing cGMP degradation.

Drug	Brand	Onset	Duration	Notes
Sildenafil	Viagra	30–60 min	Up to 12 h	Most widely used; affected by food
Tadalafil	Cialis	30–120 min	Up to 36 h ("weekend pill")	Not affected by food; also used for BPH
Vardenafil	Levitra	30–60 min	Up to 10 h	Most potent (Ki = 0.1 nM); affected by food
Avanafil	Stendra	15–30 min	Up to 6 h	Fastest onset; highest oral bioavailability (70%)

Requirements: Intact parasympathetic innervation and sexual stimulation — these drugs do NOT work without arousal.

2. Other Pharmacological Options (Second/Third Line)

Alprostadil (PGE1 analogue)

Causes cavernosal smooth muscle relaxation via cAMP pathway
Given as intracavernosal injection (Caverject) or intraurethral pellet (MUSE)
Works even with damaged innervation (unlike PDE5 inhibitors)

Alpha-Adrenoceptor Antagonists

Phentolamine (intracavernosal) — blocks α-adrenergic vasoconstriction in penile vasculature

Yohimbine (α₂ antagonist)

Historically used but now considered of dubious value and largely discredited

Key Contraindications & Warnings

Nitrates (e.g., nitroglycerin, isosorbide): Absolute contraindication with PDE5 inhibitors — dangerous hypotension. Wait at least 6 hours between a nitrate dose and a PDE5 inhibitor.
Priapism risk: Erection lasting >4 hours requires urgent medical attention (risk of ischemic damage).
Color vision changes: Sildenafil and vardenafil can cause blue-green color disturbances (PDE6 inhibition in retina).
CYP3A4 interactions: Protease inhibitors, erythromycin, and cimetidine raise PDE5 inhibitor blood levels.

Special Notes

PDE5 inhibitors are ineffective in men with spinal cord injury or complete loss of parasympathetic innervation.
Sildenafil and tadalafil are also approved for pulmonary arterial hypertension.
Tadalafil (low daily dose) is approved for benign prostatic hyperplasia (BPH) as well.

Important: Erectile dysfunction is often a biomarker of cardiovascular disease (Goldman-Cecil Medicine). A proper medical evaluation is recommended before starting treatment, especially to assess cardiac risk.

Sources: Goodman & Gilman's The Pharmacological Basis of Therapeutics; Katzung's Basic and Clinical Pharmacology, 16th Edition

Not erectile dysfunction

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