Cialis (tadalafil) will help you to treat erectile dysfunction - but it will remain efficient only as long as you continue taking it. If you are allergic to the active ingredient of this medicine - tadalafil, you will be unable to take this medicine as the same reaction will most probably happen again. If your health care provider advised you to refrain from sexual activity make sure you follow these recommendations. If during sex you experience dizziness, chest pain or fainting call your health care provider as soon as possible to prevent the situation from getting worse. Permanent vision loss is a very rare but serious side effect of taking Cialis. This may be related to the reduced blood flow to the eye nerve. Sudden loss of hearing is another serious side effect that can be rarely experienced.