Interestingly, Cialis has been more linked to improved self-esteem and other psychological factors than the other PDE5 medications. It works to treat erectile dysfunction by increasing blood flow to the penis during sexual stimulation. Cialis stays in your system for up to 36 hours and may provide support for erections during that entire time. The typicaldose of Viagra is 50 mg. Which Is More Effective? Your doctor will help you decide the best time for you to take tadalafil before sexual activity. Viagra generally only stays in your system for four to six hours.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. How Cialis Works. Tadalafil and other PDE5 inhibitors primarily work by inducing muscle relaxation. Tadalafil stops the breakdown of a chemical responsible for relaxing muscles. In ED and PAH, the drug targets muscle cells in blood vessels. When the blood vessel muscle cells relax, this expands the vessel and increases blood flow; this . For the as-needed dose, plan to take Cialis at least 30 minutes before sexual activity, but bear in mind it may take as long as 2 hours to take effect. Also, Cialis requires sexual stimulation to.