Viagra is a brand-name prescription drug that contains the active ingredient sildenafil. It is used to treat erectile dysfunction (ED), which is the inability to have or maintain an erection, in men aged 18 years and older. Viagra works by increasing blood flow to the penis, which helps a man get and keep an erection. However, it only works if the person is sexually aroused.

Viagra is also sometimes used to treat pulmonary arterial hypertension (PAH), which is high blood pressure in the blood vessels that supply the lungs. In this case, sildenafil works by relaxing blood vessels in the chest, which increases blood flow to the lungs and reduces the workload of the heart.

It is important to note that Viagra does not protect against sexually transmitted diseases (such as HIV, hepatitis B, gonorrhea, syphilis) and should not be taken with any other medications for ED or PAH, including tadalafil (Cialis), avanafil (Stendra), vardenafil (Levitra, Staxyn), alprostadil (Muse, Caverject, Caverject Impulse, Edex), or natural or over-the-counter ED treatments such as l-arginine and yohimbine.

Viagra is available as standard tablets, chewable tablets, and as a liquid that you swallow. It usually takes 30 to 60 minutes for sildenafil to work for ED, and you can take it up to 4 hours before you want to have sex. The most common side effects are headaches, feeling sick, indigestion, and dizziness, but many people have no side effects or only mild ones. It can be dangerous to take sildenafil if you also take medicines called nitrates (often given for chest pain) .