Males between the ages of 30 and 65 would be eligible to buy four tablets after a consultation with a pharmacist. If you become pregnant while taking sildenafil, call your doctor. Follow the manufacturer’s directions to use and clean the oral syringe. Oral medicines for ED usually work well and cause few side effects. Sildenafil may cause side effects.Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or alcohol. Its effects often last for 4 to 5 hours. It might work longer if you have mild to moderate erectile dysfunction. Vardenafil. It’s a prescription drug that’s used in adult males* with ED. With ED, you have trouble getting or keeping an erection. To find out more about how Viagra is used for ED, see the “ What is.  · 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.