VIAGRA : IS IT A WONDER DRUG ?
Section snippets
Chemical Structure
The chemical structure of the drug according to International Drug Nomenclature is 5-[2-Ethoxy-5-(4-methylpiperazin-1-sulphony1)phenyl]-1-methyl-3-propyl-6,7-dihydro-1H-pyrazolo [4,3-d]pyrimidin-7-one (Fig-1).
Anatomy And Physiology of Erectile Function
The penis has two corpora cavernosa, which is surrounded by a thick fibrous sheath, the tunica albuginea. The erectile tissue consists of multiple, interconnected lacunae lined by vascular epithelium. Multiple helicine arteries branch off from cavernosal artery and open directly into these lacunar spaces. Blood is drained off through subtunical venules, which are compressed against thick tunica albuginea during erection and helps in maintaining tumescence [3]. Erection is a vascular event under
Pharmacological Action
Sildenafil is a potent and competitive inhibitor of cGMP specific PDE5, predominant isoenzyme in the human corpus cavernosum and eye [8]. It decreases catabolism of cGMP, which enhances relaxation of the cavernosal smooth muscle, increases blood flow leading to increased intracavemosal pressure and penile rigidity (Fig-2). Sildenafil restores natural erectile response to various sexual stimuli but does not cause erection in the absence of stimuli. It is given in the dose of 50-100 mg orally. It
Therapeutic Effect
It is effective in erectile dysfunction of diverse origin, however it requires patent vascular system to be effective. Hence this is not effective in severe arterial or venous insufficiency. Boolell et al [8, 9] first reported its effectiveness in a double blind placebo controlled, four way cross over trial in erectile dysfunction of psychogenic origin in twelve patients. Goldstein et al [10] have demonstrated its effectiveness in men with erectile dysfunction of diverse causes e.g. organic,
Adverse Events
This drug is well tolerated. Its main adverse events are headache (16%), flushing 10%), dyspepsia (7%), diarrhoea (4.9%), nasal congestion (4%), indigestion (%), visual disturbances (3%), dizziness and rash (2%) [9, 10]. Patients reported seeing blue haze and transient increased brightness when taking doses over 200 mg. This may cause difficulty in distinguishing colour, hence pilots should be restrained from consuming this drug six hours before flying [16]. Ventricular tachycardia occurred in
Drug Interaction
Sildenafil enhances the muscle relaxant effect of nitric oxide, hence it potentiates blood pressure lowering effect of organic nitrates. 6 deaths have been reported by US FDA in patients taking nitrates, however no causal link has been established [11]. The manufacturing company has issued a warning that patients with ischemic heart disease taking nitrates should be prescribed sildenafil with caution. Drugs, which inhibit cytochrome P450 system, will increase the serum concentration of
Ethical Issue
It is likely to be prescribed by a primary care physician without complete evaluation of patient on complaint of impotence. Hence the ethical question of who should prescribe this drug should be addressed by medical fraternity by consensus and proper guidelines formulated to avoid misuse of sildenafil. In India drugs are commonly sold on prescription of varied forms of medical practitioners and even without prescription. Already there is news of availability of this drug under the counter and
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