Mechanism of action[ edit ] Penile erection during sexual stimulation is caused by increased penile blood flow resulting from the relaxation of penile arteries and the smooth muscle of the corpus cavernosum. We considered cost, medications available, and whether generic versions were offered to help reduce costs. Those who don’t feel comfortable discussing ED with an in-person provider or lack time to visit a doctor’s office may find online ED prescriptions beneficial. Contact your medical provider if you experience any side effects during treatment. For this purpose, one dose works for up to 72 hours.For the as-needed dose, plan to take Cialis at least 30 minutes before sexual activity, but bear in mind it may take as long as 2 hours to take effect. Also, Cialis requires sexual stimulation to. heart attack symptoms – chest pain or pressure, pain spreading to your jaw or shoulder, nausea, sweating. Common tadalafil side effects may include: headache; flushing (warmth, redness, or tingly feeling); nausea, upset stomach; stuffy nose; or. muscle pain, back pain, pain in your arms, legs, or back. This is not a complete list of side. 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.