Some common side effects include headaches, back pain, nasal congestion, or flushing. This allows the smooth muscles in the penis to relax and fill with blood. This increase in blood flow results in a firmer, longer-lasting erection. Brand names Why is this medication prescribed? Cialis was also investigated in combination with finasteride, another BPH medication, in another trial.milligrams (mg) 5 mg 10 mg 20 mg The cost for these different strengths depends on whether you have insurance, your individual plan coverage, and your . 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 . Tadalafil is in a class of medications called phosphodiesterase (PDE) inhibitors. It works to treat erectile dysfunction by increasing blood flow to the penis during sexual stimulation. This increased blood flow can cause an erection. Tadalafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow more easily.