The ingredient of Viagra is sildenafil, which is mainly used to treat erectile dysfunction in men. The most common side effects include causing headache, flushing of the face, indigestion, and abnormal vision, nasal congestion, back pain, muscle pain, as well as nausea, dizziness, and rash. Viagra can also have an effect on the eyes, and if there is a sudden loss of vision in one or both eyes, use of this drug should be stopped immediately. In patients who already have cardiovascular disease, there is a potential risk to the heart from taking the drug during sexual activity. If symptoms such as angina, dizziness, or nausea occur at the beginning of sexual activity, sexual activity needs to be discontinued at this time. Taking Viagra may cause symptoms of hypotension if taken with antihypertensive medications, including alpha-blockers such as doxazosin, terazosin, and prazosin. A small number of patients taking Viagra have reported erection duration, which can be prolonged for more than 4 hours, or painful erections for more than 6 hours, and should be seen immediately if the erection persists for more than 4 hours. If abnormal erections are not treated immediately, penile tissue may be damaged, which may lead to permanent loss of erectile function. It is important to avoid taking Viagra at the same time as nitrate drugs, including isosorbide mononitrate, isosorbide nitrate, and nitroglycerin, etc. The risk of sudden cardiac death may occur when taking these drugs at the same time, so it is important to prohibit taking them at the same time.