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. However, it stays in your body for up to 36 hours and can be taken every 24 hours, so it can work for you constantly. Insurance can help you save even more on consultations and medications. However, eating food, particularly high-fat food, can reduce the effectiveness of Viagra. National Library of Medicine.The dose of Cialis you need will depend on whether you’re taking it daily or as needed. Daily doses are often between and 5 milligrams (mg). The as-needed dose is usually 10 mg but can range from 5–20 mg. The typicaldose of Viagra is 50 mg. However, the dosage can range from 25– mg. Quantity 5 6 10 15 30 90 No data.  · The typical dose for once-daily Cialis for ED is milligrams (mg) to 5 mg. When used as needed for ED, the usual Cialis dose is 10 mg (with a maximum daily dose of 20 mg). The typical dose of.