Viagra, also known as sildenafil tablets, generally has few side effects if taken 3 times a month as prescribed by your doctor. Sildenafil tablets are prescription medications and require a doctor’s diagnosis before they can be used. For most patients, the medication should be taken as needed about 1 hour before sexual activity, up to 1 time per day. If you take it exactly at the dosage your healthcare provider tells you to take it three times a month, you are not exceeding its maximum dosage, so the side effects are not significant. The most common side effects of sildenafil tablets are: headache, facial flushing, stomach upset, abnormal vision such as changes in visual color (e.g., a blue tint) and blurred vision, stuffy or runny nose, back pain, muscle pain, nausea, dizziness, and rash. Less common side effects: persistent erection (abnormal erection). If a persistent erection lasts longer than 4 hours, seek medical help immediately. If not treated immediately, persistent erections may penis produce permanent damage. When an adverse reaction occurs, stop taking it immediately and seek medical attention. The drug is prohibited in combination with nitrates, it is contraindicated in women, people who are allergic to the product, children, people whose recent cardiovascular state does not allow them to have sex. The drug should be used under medical supervision.