Viagra (sildenafil) is a prescription drug used to treat ED (erectile dysfunction) in patients of any ages no matter what their problems have been caused by. Erectile dysfunction affects millions of men in the world, and about half of these patients have this condition because of certain medial conditions. Impotence can be caused by medical conditions such as diabetes or some circulatory conditions, but Viagra is expected to treat the impotence caused by any of them. If you have taken Viagra in spite of your allergic reaction to it in the past - you can get an allergic reaction and experience symptoms as trouble breathing, severe dizziness, rash, itching and unusual swelling. In that case report the symptoms to your local emergency center and do not take another dosage of Viagra until told so by your doctor. If you are currently taking a nitrate drug (usually prescribed for chest pain), make sure you do not start taking Viagra and never combine these medicines, as it can cause a serious drop in blood pressure. As a result, you may suffer a stroke, heart attack or faint. If you get dizzy or nauseous during sexual activity, or experience chest pain, stop taking Viagra and seek emergency medical help. Never take Viagra more often than every 24 hours, as it may lead to an overdose of this medication. Viagra is very well tolerated by most patients taking it and only a few side effects are possible, including increased light sensitivity, nosebleeds, difficulty falling asleep, heartburn, numbness or tingling in the arms, hands, feet, or legs, headache, muscle pain, diarrhea, flushing, and some other ones. These side effects are unlikely to persist or repeat as your body adjusts to the dosage prescribed. If the reaction of your body does not seem to improve - talk to your health care provider about a dose adjustment, as maybe you will be able to benefit from less of this medication and the side effects will disappear. In some cases more serious side effects are also possible, such as sudden loss of vision, priapism, chest pain, sudden loss of hearing, dizziness, blurred vision, lightheadedness, ringing in the ears, shortness of breath, fainting, itching or burning during urination, and rash. If the serious side effects mentioned appear, report them as soon as possible to your health care provider to prevent the situation from getting worse.