If too much is used, the chance of side effects is increased. If you see air bubbles in the syringe, repeat the steps. Common ones include: Flushing of the skin. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. It might work longer if you have mild to moderate erectile dysfunction. Read and follow these instructions carefully before you start using sildenafil and each time you get a refill of your medicine.VIAGRA can cause serious side effects. Rarely reported side effects include: an erection that will not go away (priapism). If you have an erection that lasts more than 4 hours, get . 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 . The U.S. Food and Drug Administration has approved the use of sildenafil (brand name Viagra) only in “men” older than 18 years. Pfizer, the company that manufactures this .