Viagra generally refers to sildenafil citrate, and normal men who take sildenafil citrate only once usually have no side effects, but it is impossible to generalize due to individual variability. And the drug is not recommended for normal men. Sildenafil citrate is a medication used to treat erectile dysfunction in men. Taking Sildenafil Citrate only once usually does not cause side effects, but due to individual constitution, some people may experience side effects such as headache, flushing, indigestion, diarrhea and anemia. However, the drug is not recommended for men if they do not have erectile dysfunction. It should be noted that the combination of sildenafil with nitrates is prohibited, and the drug is contraindicated in women, people who are allergic to the product, children, and people whose recent cardiovascular state does not allow them to have sex. Patients need to consult a doctor in time if they experience any unusual discomfort while taking sildenafil citrate.