Who might need a lower Viagra dose or extra monitoring? As Viagra can also make some people feel dizzy or lightheaded, this could be made worse if you drink alcohol. You should still be able to get an erection for up to five hours after taking a dose. If you’ve tried taking it several times and still haven’t been able to get a hard enough erection you should consult your doctor, because a higher dose may work for you. Just because a side effect is stated here doesn’t mean that all people taking Viagra will experience that or any side effect.Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles . What does Viagra do for ED? Viagra works to treat ED by helping you have and maintain an erection. The drug does this by improving blood flow into your penis when . The average length of time Viagra lasts. On average, Viagra lasts between 2 and 3 hours before its effects start to diminish. Viagra can last up to 5 hours or longer depending on .