You need a prescription from a doctor to buy tadalafil. Check the healthdirect’s medicines section for more information about tadalafil. If you have an erection that lasts for more than 4 hours, you should go to the nearest hospital emergency department. The decision of availability is completely up to the company marketing the product. After taking a single tablet, some of the active ingredient of Cialis remains in your body for more than 2 days.The normal dosage of Cialis for occasional use in treating ED is 10 mg taken before sexual activity. If you have side effects with the mg dose, your doctor may prescribe a 5-mg dose. If the Cialis (tadalafil) is a prescription medication for those with erectile dysfunction. Many providers sell Cialis online — learn about the safest and most discreet providers here. Save. Cialis and Viagra are not sold over the counter. You need a prescription. You may see a doctor and get a prescription, or try and buy with no prescription in a farmacia. .