This medicine also works best when you take it an hour before sex on an empty stomach. Oral medicine for ED also might not be safe or work well if you have: Very low blood pressure, called hypotension, or uncontrolled high blood pressure, called hypertension. You can take it in a small dose daily or in a larger dose as needed. It lasts 4 to 5 hours, or longer if you have mild to moderate ED. Other ED treatments include Cialis, which is powered by the active ingredient Tadalafil. These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects.It’s best to take sildenafil (Viagra) about 1 hour before sex, but you can take it anywhere between 30 minutes and 4 hours before you need it. You can take sildenafil . Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in . By Ginger Skinner Last updated: June 14, If you’re looking for a less costly Viagra, you might want to ask your doctor about sildenafil. It’s the active ingredient .