Viagra vs. Mechanism of action[ edit ] Penile erection during sexual stimulation is caused by increased penile blood flow resulting from the relaxation of penile arteries and the smooth muscle of the corpus cavernosum. Cialis stays in your system for up to 36 hours and may provide support for erections during that entire time. Talk with your healthcare provider about which might be best for you. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Just make sure you take it at the same time every day. Both Cialis and Viagra are effective at these doses.Dosage. 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. Cart. Transfer your prescription for Cialis (Brand for Tadalafil, Oral Tablet) on Amazon Pharmacy. FREE SHIPPING for Prime members. The drug comes in low-dose versions that can be taken daily. This means you can have sex at any time, rather than needing to take a pill from time to time as needed like other ED brands. The starting dose for daily-use Cialis is milligrams (mg). If that doesn’t work, you can increase your daily dose up to 5 mg.