PDE5 normally causes blood vessels in the penis to return to their normal size, ending an erection. Tadalafil works for these conditions by promoting muscle relaxation, which improves blood flow in PAH and urinary flow in BPH. It may be taken with or without food. How exactly does Cialis work? Though this growth is not cancerous, prostate cancer and BPH can co-occur. · Taking Cialis depends on why you need it. For erectile dysfunction (ED), as needed, start with 10 mg before sexual activity, and your doctor may adjust it to 20 mg or 5 mg. For daily use, take mg daily, possibly increased to 5 mg. 1. If you’re taking Cialis for benign prostatic hyperplasia (BPH), the standard dose is 5 mg taken. 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. Cialis is a prescription medication containing the active ingredient tadalafil. Cialis belongs to a group of medications known as phosphodiesterase-5 (PDE-5) inhibitors. Healthcare providers commonly prescribe tadalafil to treat erectile dysfunction (ED).