Cialis is the trade name and generic name for tadalafil tablets, which are generally harmless when taken in moderation once a week as prescribed by your doctor, but may bring about some adverse effects, most commonly headaches and indigestion. Tadalafil is a type 5 phosphodiesterase inhibitor and belongs to the same class of therapeutic drugs as sildenafil, which is used to improve penile hardness. Adverse reactions such as headache and indigestion are possible when taking tadalafil. Some patients may also experience back pain, myalgia, nasopharyngitis, nasal congestion and facial flushing, etc. Patients who are allergic to this drug are prohibited from using it. The therapeutic effect of tadalafil lasts for about 36 hours, and the maximum dosage frequency is once a day. Patients are advised to seek medical attention in case of discomfort.