If you become dizzy or nauseated, or have pain, numbness, or tingling in your chest, arms, neck, or jaw during sexual activity, stop and call your doctor right away. You could be having a serious side effect of sildenafil. Stop using sildenafil and get emergency medical help if you have sudden vision loss. Get emergency medical help if you have any of these signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat.
Stop using sildenafil and call your doctor at once if you have any of these serious side effects:
chest pain or heavy feeling, pain spreading to the arm or shoulder, nausea, sweating, general ill feeling;
swelling in your hands, ankles, or feet;
shortness of breath;
feeling light-headed, fainting; or
penis erection that is painful or lasts 4 hours or longer.
Continue taking sildenafil and talk with your doctor if you have any of these less serious side effects:
warmth or redness in your face, neck, or chest;
upset stomach; or
Side effects other than those listed here may also occur. Talk to your doctor about any side effect that seems unusual or that is especially bothersome.