1, Viagra is a vasodilator, which has high selectivity for penile cavernous blood vessels, but also has effects on other blood vessels. If it acts on facial blood vessels, it may cause adverse reactions such as facial congestion, blushing and head swelling, and the overall incidence rate is low; 2. Some people may have symptoms such as gastrointestinal reaction and palpitation, but the overall incidence is relatively low. 3. If you have other basic diseases, such as heart disease, hypertension or prostatic hyperplasia, and take antihypertensive drugs, prostate α receptor blockers and other drugs, when taking Viagra, you should pay attention to the interaction between these drugs and Viagra, which will cause blood pressure to drop. 4. Pay attention to the contraindications of Viagra, and don't take it together with nitroglycerin once every 1-2 days.