Cialis stays in your system for up to 36 hours and may provide support for erections during that entire time. Cialis vs. Recent heart or blood vessel problems. Though this growth is not cancerous, prostate cancer and BPH can co-occur. Viagra is always taken as needed, usually an hour before having sex. These are commonly prescribed for an enlarged prostate or male pattern baldness.Cialis and Viagra have both been shown to be safe and effective for erectile dysfunction. The main differences are cost and timing. For occasional use, most people start with Viagra which also tends to be cheaper. Cialis lasts longer and can also be taken daily at a . Only 5% of males between the ages of 20 and 39 report ED symptoms, while over 70% of males over 70 report difficulty having or maintaining an erection. The most commonly prescribed medications to treat ED are PDE5 inhibitors like Cialis (tadalafil) and Viagra (sildenafil). 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.