precaution-medicine-erection-treatment

VIAGRA(R) and similar medicines that contain Sildenafil Citrate are called PDE5 inhibitors.
SILDENAFIL
TADALAFIL
VARDENAFIL
are also PDE5 inhibitors

PDE5 inhibitors should be used with caution if you have any of the following conditions:

Recent stroke
Severe heart disease, such as unstable angina, a recent heart attack, or arrhythmia
Severe heart failure
Uncontrolled hypertension (high blood pressure)
Uncontrolled diabetes
Very low blood pressure (hypotension)

http://www.nlm.nih.gov/medlineplus/ency/article/003164.htm