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. Cialis Warnings Before you begin using a medication, be sure to inform your doctor of any medical conditions or Allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. The as-needed dose is usually 10 mg but can range from 5—20 mg. Both Cialis and Viagra are effective at these doses. Summary Cialis and Viagra are both pills for treating erectile dysfunction. They are PDE5 inhibitors that allow for increased blood flow to the penis, leading to more frequent and harder erections. Viagra generally only stays in your system for four to six hours.Cialis (tadalafil) is a brand-name prescription medication. The Food and Drug Administration (FDA) has approved it for use in adult males.* It’s approved to: treat erectile dysfunction (ED). The lowest price on test1.ru for generic Cialis (tadalafil) 20 mg is $ per tablet for 88 tablets at PharmacyChecker-accredited online pharmacies. Tadalafil International Price Comparison Highlight. Lowest U.S. Pharmacy Coupon Price: $ per tablet for 90 tablets. Brand Cialis. Generic Tadalafil. No data.. Form. Bottle. Box. Disp Pack. Tablet. No data.. Dosage. mg. 5mg. 10mg. 20mg. No data.. Quantity. 5. 6. No data.