If you have certain health conditions or are taking certain medications, your doctor may tell you to take tadalafil less often or may prescribe a lower dose to be taken once a day. Most erectile dysfunction medications belong to the drug class known as PDE5 inhibitors , which help relax the muscles and increase blood flow to allow for a full erection. That means seven out of 10 people who take these drugs as prescribed will have more frequent or longer-lasting erections. When a man is sexually stimulated, his body’s normal response is to increase blood flow to his penis to produce an erection. Online pharmacies such as GoodRx are also helping bring down the costs of erectile dysfunction medications.Cialis is a prescription drug used to treat erectile dysfunction and benign prostatic hyperplasia. Learn how to lower long-term drug costs and more. Side Effects. Headache, stomach upset, back pain, muscle pain, stuffy nose, flushing, or dizziness may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly. To. 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 .