Follow your doctor’s directions carefully, and ask your doctor or pharmacist to explain any part you do not understand. Some people buy tadalafil online without a prescription. 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. Continue to take tadalafil even if you feel well. Resources and support See the Healthy Male site for more information about erectile dysfunction and its treatment. Your doctor may need to change the doses of your medications or monitor you carefully for side effects. Benign prostatic hypertrophy is a condition in which the prostate gland becomes enlarged, obstructing the free flow of urine.4 rows · Both treatment options are available in doses of 5, 10, and 20 mg. The costs are: Cialis: A. How much does tadalafil or cialis cost near you? Find the best tadalafil or cialis prices near you now with our cost comparison tool and start saving today. Cialis lasts up to 36 hours in the body, while Viagra lasts 4–6 hours in the body. Both are taken 30 minutes before sex. Cialis can cause limb pain, while Viagra can cause vision change, rash.