If you have heart problems severe enough to make sexual activity a danger, you should avoid using Viagra. Use it cautiously---if at all---if you've had a heart attack, stroke, or life-threatening heart irregularities within the past 6 months. Be equally cautious if you have severe high or low blood pressure, heart failure, or unstable angina (crushing heart pain that occurs at any time).
If you take Viagra and develop cardiac symptoms (for example, dizziness, nausea, and chest pain) during sexual activity, do not continue. Alert your doctor to the problem as soon as possible.
If you have a condition that might result in long-lasting erections, such as sickle cell anemia, multiple myeloma (a disease of the bone marrow), or leukemia, use Viagra with caution. Also use cautiously if you have a genital problem or deformity such as Peyronie''s disease. If an erection lasts more than 4 hours, seek treatment immediately. Permanent damage and impotence could result.
If you have a bleeding disorder, a stomach ulcer, or the inherited eye condition known as retinitis pigmentosa, use this medication with caution. Its safety under these circumstances has not yet been studied.
Remember that Viagra offers no protection from transmission of sexually transmitted diseases, such as HIV, the virus that causes AIDS.
This drug is not for use by women. Its effects during pregnancy have not been studied.

Side effects cannot be anticipated. If any develop or change in intensity, inform your doctor as soon as possible. Only your doctor can determine if it is safe for you to continue taking Viagra.

Some side effects may include :
Abnormal vision (color tinge, blurring, sensitivity to light),acid indigestion, diarrhea, flushing, headache, nasal congestion, urinary tract infection

Less common to rare side effects may include :
Abdominal pain,abnormal dreams, abnormal ejaculation, allergic reactions, anxiety, asthma, bloodshot eyes, bone pain, breast enlargement, cataracts, chest pain, chills,coordination problems, cough,depression, difficulty breathing, difficulty swallowing, dilated pupils, dizziness, drowsiness, dry eyes, dry mouth, emotional or mental disturbances, eye inflammation or pain, other eye disorders, fainting,falling, genital problems, gout, gum inflammation, heart problems, increased night-time urination, increased pressure in the eyes, insomnia, itchy skin, joint disease, light sensitivity, loss of bladder control (urinary incontinence), low blood pressure, migraine headache, muscle ache, numbness, oral inflammation, pain, painful erection, prolonged erection, raised skin patches, rapid or throbbing heartbeat, rectal bleeding, respiratory inflammation, ringing in the ears, seizure, sinus and throat inflammation, skin rash, skin ulcer, slow reflexes, stomach or intestinal inflammation, sweating, swelling, thirst, tremor, vomiting, weakness

Rare but serious may include:
Heart attack, stroke, heart irregularities, dangerous surges in blood pressure, and sudden death have all been reported after use of Viagra, usually in men with existing cardiac risk factors, and typically during or shortly after sex. Also blindness is now been reported as another side effect but no definite link has been established yet but it could be linked so always, always consult a physician before the use of any medications and get constant check ups to ensure you are right for viagra.