What is sildenafil, and how does it work (mechanism of action)?
Viagra (sildenafil) is an oral drug that is used for treating
impotence (or erectile disfunction, ED, the inability to attain or maintain a penile erection). It is in a class of drugs called phosphodiesterase inhibitors (PDE-5 inhibitors) that also includes
- tadalafil (Cialis),
- vardenafil (Levitra, Staxyn ODT), and
- avanafil (Stendra).
It has been estimated that impotence affects 140 million men worldwide. Over half of all men with impotence are thought to have some physical (medical) cause. The remainder are believed to have psychogenic causes of impotence. Medical causes of impotence include diabetes and circulatory, neurological, or urological conditions.
Penile erection is caused by the engorgement of the penis with
blood.
This engorgement occurs when the blood vessels delivering blood
to the
penis increase the delivery of blood and the blood vessels
carrying blood
away from the penis decrease the removal of blood. Under normal
conditions, sexual stimulation leads to the production and
release of
nitric oxide in the penis. Nitric oxide then activates the
enzyme,
guanylate cyclase, which causes the production of cyclic
guanosine
monophosphate (cGMP). It is the cGMP that is primarily
responsible for the
erection by affecting the amount of blood that the blood
vessels deliver
and remove from the penis.
Viagra inhibits an enzyme called phosphodiesterase-5
(PDE5) which
destroys the cGMP. Thus, Viagra prevents the destruction of
cGMP and
allows cGMP to accumulate and persist longer. The longer cGMP
persists,
the more prolonged the engorgement of the penis. The FDA approved Viagra in
March 1998.
What brand names are available for sildenafil?
Viagra
Is sildenafil available as a generic drug?
GENERIC AVAILABLE:Yes
Do I need a prescription for sildenafil?
Yes
What are the side effects of sildenafil?
Approximately 15% of persons taking
Viagra
experience side effects. The most common side effects are:
- facial flushing (1 in 10),
- headaches (1 in 6),
- stomach pain,
- nasal congestion,
- nausea,
- dizziness,
- rash,
- urinary tract infections (UTIs)
- diarrhea, and
- an inability to differentiate between the colors green and blue.
What is the dosage for sildenafil?
- The usual recommended dose is 25 to 100 mg 1 hour before sexual activity.
- The maximum dose is 100 mg daily.
- The elderly (over 65 years of age) should start at 25 mg before sexual activity.
- Sildenafil is rapidly absorbed. Maximum observed plasma concentrations are reached within 30 to 120 minutes (median 60 minutes) of oral dosing in the fasted state. When sildenafil is taken with a high fat meal, the rate of absorption is reduced, with an average delay in the time to maximal concentration of 1 hour.
Which drugs or supplements interact with sildenafil?
Viagra increases the
effects
of the blood pressure lowering medications. It also increases the blood pressure lowering effects of nitrates,
for example, isosorbide
dinitrate
(Isordil), isosorbide mononitrate (Imdur, Ismo, Monoket),
nitroglycerin
(Nitro-Dur, Transderm-Nitro) that are used primarily for treating angina. Patients taking nitrates should
not receive
Viagra.
Patients should not combine Viagra with other PDE5 inhibitors (for example, vardenafil [Levitra], tadalafil [Cialis]).
Cimetidine (Tagamet), erythromycin, ketoconazole (Nizoral),
itraconazole
(Sporanox), atazanavir (Reyataz), and mibefradil (Posicor) can cause marked increases
in the
amount of Viagra in the body. Patients taking these
medications should
be observed carefully if sildenafil is used.
It is expected that rifampin will decrease blood levels of
Viagra
and probably reduce its effectiveness.
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Is sildenafil safe to take if I’m pregnant or breastfeeding?
Although extensive testing in animals has
demonstrated no negative effects on the fetus, Viagra has
not been
studied in pregnant women. There is no effect on sperm count or
motility
of sperm in men.
It is not
known whether sildenafil is excreted into
breast milk.
What else should I know about sildenafil?
What preparations of sildenafil are available?
Sildenafil (Viagra) is a
blue, film-coated, rounded, diamond-shaped tablet that comes in 25, 50, and 100
mg tablets for treating impotence.
How should I keep sildenafil stored?
Tablets should be kept at room temperature between 15 C and
30 C
(59 F and 86 F).