Viagra (sildenafil) can be recommended by your health care provider if you have erectile dysfunction and need to be taking some drug to make the erection last longer. You will need to make sure your health care provider is aware of the fact you are using other drugs, especially any of the following ones: nevirapine, alpha blockers, efavirenz, other erectile dysfunction meds, medications for high blood pressure, barbiturates, cimetidine, seizure medications, beta blockers, amlodipine, anticoagulants, rifabutin, HIV protease inhibitors, erythromycin, rifampin, certain antifungals, or bosentan. Viagra is taken half an hour before having sex, although some people may need longer to feel the effects of this medicine. Make sure you do not take more of Viagra than necessary, as a higher dose is not going to give you a harder erection and can produce side effects. The following side effects are possible when Viagra is taken: light sensitivity, changes in color vision, nosebleeds, diarrhea, flushing, insomnia, muscle aches, headache, heartburn, numbness, burning, and tingling.