Safety of combined use of PDE5 inhibitors and alpha-blockers may be affected by other variables, including intravascular volume depletion and other antihypertensive drugs. Tadalafil may cause side effects. Do not stop taking tadalafil without talking to your doctor. Tadalafil is sometimes taken as needed, usually at least 30 minutes before sexual activity and not more often than once every 24 hours. This increased blood flow can cause an erection.Strengths: mg, 5 mg, 10 mg, and 20 mg; Brand: Cialis. Form: oral tablet; Strengths: mg, 5 mg, 10 mg, and 20 mg; Adult dosage (ages 18–64 years) Tadalafil can be used either as needed or. Tadalafil (Cialis) is available both as brand-name and generic tablets. Common side effects of this medication include headache and indigestion. Tadalafil But your blood pressure shouldn’t drop by too much. In clinical studies, people who took the 20 mg dose of tadalafil (Cialis) experienced an average drop in blood pressure by about 2 . Tadalafil (Cialis) is a phosphodiesterase-5 enzyme (PDE-5) inhibitor. It works by blocking PDE This is a protein in your body that breaks down the molecules that normally cause an erection. When it blocks PDE-5, tadalafil (Cialis) helps an erection occur by allowing certain muscles in the penis to relax and the penis to fill with blood.