Do not start these medications while taking sildenafil without discussing it with your healthcare provider. Often, these are prescribed for chest pain called angina. Ask your pharmacist for a list of the ingredients. Do not mix the liquid with other medications or add anything to flavor the medication. Rarely reported side effects include: an erection that will not go away priapism.Side Effects. Dizziness, headache, flushing, or stomach upset may occur. Vision changes such as increased sensitivity to light, blurred vision, or trouble telling blue and green . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes . Viagra is used to treat erectile dysfunction (ED). It decreases ED symptoms by helping you get and keep an erection. Erections happen when a protein called cyclic .