Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. Tadalafil Cialis. These are commonly prescribed for an enlarged prostate or male pattern baldness. These medicines all work well for ED. Medicines that you take by mouth are called oral medicines.an erection that will not go away (priapism). If you have an erection that lasts more than 4 hours, get medical help right away. If it is not treated right away, priapism can . Typical dosing for Sildenafil (Viagra) The typical dose is to take 50 mg of sildenafil (Viagra) by mouth, about 1 hour before sex, as needed. The maximum . Viagra comes as tablets that you take by mouth, when needed. Available strengths: 25 mg, 50 mg, mg. Viagra comes in these strengths: 25 milligram (mg), .