Patients should seek prompt medical attention if their erection lasts longer than four hours. Erectile Dysfunction, though, is when your difficulty with getting an erection or staying hard becomes a regular occurrence. Tadalafil does not prevent pregnancy or the spread of sexually transmitted diseases such as human immunodeficiency virus HIV. If you have certain health conditions or are taking certain medications, your doctor may tell you to take tadalafil less often or may prescribe a lower dose to be taken once a day. In , Pfizer scientists discovered that sildenafil, which also inhibits the PDE5 enzyme, caused penile erection in males participating in a clinical study of a heart medicine.The recommended starting dose for Cialis intended for once daily use is mg, take at the same time each day, without regard to timing of sexual activity. This may be increased to 5mg/day based on need and tolerability. May be taken with or without food. Cialis is available as a generic, under the name tadalafil. 3. Roman is a telehealth prescription service and online pharmacy that provides prescription Cialis for ED and a host of other conditions, like hair loss, cold sores, and premature ejaculation. When. If you do not meet the NHS criteria for tadalafil, you may get a private prescription from a doctor. This means you’ll need to pay the full cost of the medicine. Key facts. It usually takes 30 to 60 minutes for tadalafil to work for erectile dysfunction. You can take it at least 30 minutes before you want to have sex.