Warning:
Combining Viagra with Cardura and Other Alpha-Blockers
Alert patients who take Viagra with an alpha-blocker
about possible hypotension and dizziness.
These drugs are commonly used together. Many men
who take alpha-blockers for BPH also take Viagra.
In fact, doxazosin is sometimes combined with
Viagra to increase Viagra's effect. Viagra is
a modest vasodilator. It can lower blood pressure
for several hours after a dose. Now there are
reports of postural hypotension and dizziness
in men taking Viagra within 4 hours of doxazosin.
This interaction might also occur with terazosin
Hytrin),prazosin (Minipress) and tamsulosin(Flomax).
Recently
Pfizer added a new precaution to the drug labeling
of Viagra (sildenafil) which involves a drug interaction
between Viagra and alpha-blockers, namely Cardura
(doxazosin). Patients taking both medications
have reported symptomatic hypotension.1 This interaction
is of particular importance because of the frequency
of concomitant use. Patients taking doxazosin
for benign prostatic hyperplasia oftentimes take
Viagra as well for erectile dysfunction. Incidentally,
dosing for both drugs is usually in the evening
hours.
The mechanism of action of Viagra for erectile
dysfunction involves inhibition of phosphodiesterase
type 5 (PDE5).1 PDE5 is also found in vascular
smooth muscle. It is believed that inhibition
of PDE5 in vascular smooth muscle is what causes
peripheral arterial-venous dilation and potential
blood pressure reduction. Viagra taken alone has
been reported to cause decreases in supine blood
pressure of up to 8.4/5.5 mmHg systolic/diastolic.1
This occurs mainly within a couple hours of dosing.
Its effects on blood pressure do not appear to
be dose dependent. When taken alone, these effects
are not clinically significant.
When Viagra 25 mg was given with doxazosin 4 mg
mean supine blood pressures were lowered an additional
7 mmHg systolic and 7 mmHg diastolic compared
to doxazosin alone. When the dose of Viagra was
increased above 25 mg, several patients reported
symptomatic hypotension within one to four hours
of dosing.1 Studies have not shown clinically
significant lowering of blood pressure when Viagra
is taken with other antihypertensive medications
(even when patients were taking multiple antihypertensives).2,3
One study actually describes the intentional use
of doxazosin with Viagra for patients with non-organic
erectile dysfunction (ED).4 The patients in this
placebo controlled trial had failed Viagra monotherapy
for ED. Subjects were given Viagra 100 mg plus
either doxazosin 4 mg or placebo. Patients who
took doxazosin plus Viagra showed a significant
improvement in the International Index of Erectile
Function (IIEF) questionnaire compared to those
taking placebo and Viagra. Side effects were minor
and there were no significant adverse effects
on blood pressure. However, the study size was
small, only 28 patients. It was also conducted
prior to the precautions issued by Pfizer.
Doxazosin's effect on ED could be due to the presence
of alpha1 adrenergic receptors on the corpus cavernosum.4
Blood flow to this area would be increased as
a result of reduction in vasoconstrictive sympathetic
tone. Keep in mind you may see some practitioners
using doxazosin with Viagra for ED patients not
responding to Viagra alone. Appropriate precautions
should still be taken with these patients.
Although incidences of symptomatic hypotension
have only been reported with doxazosin, Pfizer
recommends that precautions be taken with all
alpha-blockers. For patients taking Viagra and
alpha-blockers, recommend that any doses of Viagra
above 25 mg not be taken within four hours of
each other. Product information for Viagra. Pfizer.
New York, NY 10017. September 2002. Am J Hypertens
2001;14(1):70-3.
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FACT:
Cardiovascular disease is the number one cause
of death in industrialized
countries. It is also set to overtake infectious
diseases as the most common cause
of death in many parts of the less developed world.
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