Viagra works by blocking certain naturally occurring enzymes in the body, causing blood vessels to dilate and enhancing blood flow to the penis.
heart failure;angina (chest pain);a heart rhythm disorder;a heart attack or stroke;heart surgery;severe or uncontrolled high blood pressure;low blood pressure;retinitis pigmentosa (an inherited condition of the eye);hearing or vision problems, or vision loss;bleeding problems;a blood cell disorder such as sickle cell anemia, multiple myeloma, or leukemia;a stomach ulcer;liver disease, kidney disease (or if you are on dialysis);priapism (an erection that is painful or lasts longer than 4 hours);a physical deformity of the penis (such as Peyronie's disease); orif you have been told you should not have sexual intercourse for health reasons.

For most patients, the recommended dose is 50 mg taken, as needed, approximately 1 hour before sexual activity. However, VIAGRA may be taken anywhere from 30 minutes to 4 hours before sexual activity. The maximum recommended dosing frequency is once per day.
If you are taking a medicine and are worried about its effects, don't stop taking it suddenly. Instead discuss this with your doctor or pharmacist. There might be a different medicine you can take that is less likely to cause sexual problems. Read more about medicines and sexual problems. existing values or attitudes towards sex stress, guilt, depression or fatigue performance-related anxiety about your performance during sex traumatic past experiences including sexual abuse poor body image relationship problems (possible anger, power or control issues with your partner) problems with intimacy and communication intercourse less than once per week (twice the risk of developing erectile dysfunction).