Viagra (sildenafil) is a brand-name, FDA-approved prescription medication used to treat erectile dysfunction (ED) in men by relaxing muscles and increasing blood flow to the penis, usually taken 30–60 minutes before sexual activity. It belongs to the phosphodiesterase type 5 (PDE5) inhibitor class. Common side effects include headache, flushing, indigestion, and dizziness, with serious, albeit rare, risks like vision loss or priapism. 
Key Details About Viagra
What is Sildenafil Used For: Primarily for treating Erectile Dysfunction (impotency). It is also used under different brand names (e.g., Revatio) to treat Pulmonary Arterial Hypertension (PAH).
How it Works: As a PDE5 inhibitor, sildenafil helps to increase cGMP levels, which relaxes blood vessels and allows for increased blood flow to the penis, facilitating an erection only when sexual stimulation occurs.
Side Effects: The most common side effects are headache, flushing (feeling of warmth), dizziness, blurred vision, nasal congestion, and indigestion.
Drug Interactions: Crucially, Viagra must not be taken with nitrates (often prescribed for chest pain or heart issues), as this can cause a severe, life-threatening drop in blood pressure. Other interactions include alpha-blockers, some antibiotics, and HIV protease inhibitors.
Dosage & Usage: Available in 25mg, 50mg, and 100mg doses. It should be taken no more than once per day, generally 1 hour before sexual activity, and it is most effective on an empty stomach.