The side effects caused by Viagra are usually mild to moderate. They usually last no more than a few hours before dissipating. Some side effects may be more likely to occur with higher doses. The most common side effects of Viagra are:

• Headache
• Flushing of the face
• Upset stomach

Several side effects occur less often, and may include:

• Temporary vision changes with regard to color (it may be more difficult to tell the difference between blue and green objects, or some objects may appear to be more blue-tinted than their actual color)
• More sensitivity to light
• Blurred vision
• In rare instances, men taking PDE5 inhibitors (oral erectile dysfunction medicines, including Viagra) reported a sudden decrease or loss of vision in one or both eyes. It is not possible to determine whether these events are related directly to these medicines, to other factors such as high blood pressure or diabetes, or to a combination of these. If you experience sudden decrease or loss of vision, stop taking PDE5 inhibitors, including Viagra, and call a doctor right away.
• In rare instances, men have reported an erection that lasts many hours. You should call a doctor immediately if you ever have an erection that lasts more than four hours. If not treated right away, permanent damage to your penis could occur after six hours.