You might need to take Viagra on a few different occasions before it starts to work for you. Children younger than 1 year of age—Use and dose must be determined by your doctor. Your doctor or other health care professional can recommend one for you based on these factors, other health conditions you have and any other medicines you take. Side effects are uncommon and generally mild, if you experience any serious pain or side effects which last longer than hours, seek medical attention immediately. In: StatPearls [Internet]. People with hereditary degeneration of the back of the eyeball retina , such as retinitis pigmentosa. · 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.  · For treatment of erectile dysfunction: For oral dosage form (tablets): Adults up to 65 years of age—50 milligrams (mg) as a single dose no more than once a day, 1 hour before sexual intercourse. Alternatively, the medicine may be taken 30 minutes to 4 hours before sexual intercourse. Your doctor may adjust your dose if needed. Sildenafil is a medicine used to treat erection problems (erectile dysfunction). Sildenafil is also sometimes used to treat pulmonary hypertension (high blood pressure in the blood vessels that supply the lungs). If you have erection problems sildenafil works by temporarily increasing blood flow to your penis when you get sexually excited.