These medicines are called PDE5 inhibitors. There may be rare occurrences of priapism or painful, prolonged erections. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months. The patent was published in and expired in Viagra is always taken as needed, usually an hour before having sex. Who should not take Cialis?stockcam / Getty Images. What is generic Cialis? Generic Cialis is called tadalafil, which is the active ingredient in Cialis. Cialis is Food and Drug Administration (FDA) approved to treat. What is Tadalafil (Cialis)? Drug Facts. Common Brands Cialis. Drug Class PDE-5 inhibitor. Controlled Substance Classification Not a controlled medication. Generic Status No lower-cost generic available. Availability Prescription only. How Tadalafil (Cialis) works. Tadalafil (Cialis) is a phosphodiesterase-5 enzyme (PDE-5) inhibitor. Cialis Prices, Coupons & Savings Tips – MDRx. DRUG_DESCRPTION Compare DRUG_NAME prices and find coupons that could save you up to 80% instantly at pharmacies near you such as CVS, Walgreens.