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Drug Class and Mechanism: Tadalafil is an oral drug that
is used for treating impotence (the inability to attain
or maintain a penile erection). It is in a class of
drugs called phosphodiesterase inhibitors. |
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Erection of the penis is caused by the filling of the
penis with blood. Filling occurs because the blood
vessels that bring blood to the penis increase in size
and deliver more blood to the penis, and, at the same
time, the blood vessels that take blood away from the
penis decrease in size and remove less blood from the
penis. |
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Sexual stimulation that leads to an erection causes the
production and release of nitric oxide in the penis. The
nitric oxide causes an enzyme, guanylate cyclase, to
produce cyclic guanosine monophosphate (cGMP). It is the
cGMP that is primarily responsible for increasing and
decreasing the size of the blood vessels carrying blood
to and from the penis, respectively, and causing the
erection. When the cGMP is destroyed by another enzyme,
phosphodiesterase-5, the blood vessels return to their
normal size, blood leaves the penis, and the erection
ends. Tadalafil prevents phosphodiesterase-5 from
destroying cGMP so that cGMP stays around longer. The
persistence of cGMP leads to a more prolonged
engorgement of the penis with blood. |
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EDosing: For most individuals, the recommended starting
dose of tadalafil is 10 mg per day taken before sexual
activity. Depending on the adequacy of the response or
side effects, the dose may be increased to 20 mg or
decreased to 5 mg a day. The effect of tadalafil may
last up to 36 hours. Individuals who are taking
medications that increase the blood levels of tadalafil
should not exceed a total dose of 10 mg in 72 hours.
Tadalafil may be taken with or without food since food
does not affect its absorption from the intestine. |
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Drug interactions: The breakdown and elimination of
tadalafil from the body may be decreased by
erythromycin, ketoconazole (Nizoral), itraconazole (Sporanox),
indinavir (Crixivan) and ritonavir (Norvir). Therefore,
these drugs may increase the levels of tadalafil in the
blood. If these drugs are being used at the same time as
tadalafil, the dose of tadalafil should be reduced in
order to avoid side effects from high levels of
tadalafil. |
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Side Effects: The most common side effects of tadalafil
are facial flushing (reddening), headaches, stomach
upset, diarrhea, flu-like symptoms and nausea. Tadalafil
also may cause low blood pressure, blurred vision and
changes in color vision, and abnormal ejaculation.
Tadalafil has been associated with prolonged erections
or priapism (painful erections lasting more than 6
hours). |
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