These are commonly prescribed for an enlarged prostate or male pattern baldness. As with sildenafil, a high-fat meal or alcohol keep the body from absorbing it as quickly. Dosing The dose of this medicine will be different for different patients. If you experience a sudden loss of vision in one or both eyes, contact your doctor immediately. Its effects often last for 4 to 5 hours. Alternatively, the medicine may be taken 30 minutes to 4 hours before sexual intercourse. · Viagra comes as an oral tablet that contains the active drug sildenafil. Sildenafil is also available in generic form. Viagra is classified as a phosphodiesterase type 5 (PDE5) inhibitor. Dizziness, headache, flushing, or stomach upset may occur. Vision changes such as increased sensitivity to light, blurred vision, or trouble telling blue and green colors apart may also occur. If. If you have these symptoms, stop taking VIAGRA and contact a doctor right away. have or have had heart problems such as a heart attack, irregular heartbeat, angina, chest pain, narrowing of the aortic valve, or heart failure. have had heart surgery within the last 6 months. have pulmonary hypertension. have had a stroke.