Viagra generally refers to sildenafil. A response can usually be expected around 30 to 120 minutes after taking it, but it requires sexual stimulation. After sildenafil is taken, blood levels peak around 30 to 120 minutes, so its onset of action is generally within that range. However, proper sexual stimulation is necessary for the drug to work. The drug will not work without sexual stimulation after taking it. Its effects can last until about 4 hours or so after taking the drug. Adverse reactions such as facial flushing, headache, dizziness, rash, respiratory infections, back pain, nasal congestion, flu-like symptoms, joint pain, indigestion, and visual abnormalities may occur after taking this medication. This drug is contraindicated in people taking nitrates, women, people who are allergic to the product, children, and people whose recent cardiovascular status makes them unfit for sexual intercourse. The drug is a prescription drug and should be used in strict accordance with medical advice.