Medicines that you take by mouth are called oral medicines. They're often the first line of treatment for trouble getting or keeping an erection, called erectile dysfunction (ED). Oral medicines for ED usually work well and cause few side effects.

The four main medicines taken by mouth for erectile dysfunction are:

• Avanafil (Stendra).
• Sildenafil (Viagra).
• Tadalafil (Cialis).
• Vardenafil.

These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. This boosts blood flow and helps you get an erection from sexual activity.

Each oral medicine for ED has a slightly different chemical makeup. These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects. Your doctor or other health care professional can recommend one for you based on these factors, other health conditions you have and any other medicines you take.

• Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or alcohol. Its effects often last for 4 to 5 hours. It might work longer if you have mild to moderate erectile dysfunction.

• Vardenafil. This medicine also works best when you take it an hour before sex on an empty stomach. As with sildenafil, a high-fat meal or alcohol keep the body from absorbing it as quickly.

  Vardenafil usually works for 4 to 5 hours. Its effects might last longer if you have mild to moderate ED. A newer form of the medicine that dissolves on the tongue might work faster than the pill that you swallow.

• Tadalafil (Cialis). This medication is taken with or without food an hour before sex. It works for up to 36 hours. You can take it in a small dose daily or in a larger dose as needed.
• Avanafil (Stendra). You can take this medicine with or without food 30 minutes before sex, depending on the dose. It lasts 4 to 5 hours, or longer if you have mild to moderate ED.

These medicines all work well for ED. When choosing one, tell your doctor or other health care professional what your preferences are, such as cost, ease of use, how long the drug's effects last and side effects. Generic versions of sildenafil, vardenafil and tadalafil are available.

Check with your health care team before you take any medicine for erectile dysfunction. Oral ED medicines might not work or might be dangerous if you also take any of the following:

• Nitrate medicines. Often, these are prescribed for chest pain called angina. Some examples of nitrates are nitroglycerin (Nitro-Dur, Nitrostat, others), isosorbide mononitrate (Monoket) and isosorbide dinitrate (Bidil).
• Alpha-blockers. These are commonly prescribed for an enlarged prostate or male pattern baldness. They include dutasteride (Avodart, Jalyn) and finasteride (Propecia, Proscar).
• Medicines that block an enzyme called CYP3A4. These include itraconazole (Sporanox, Tolsura), ketoconazole (Extina, Ketozole, others) and ritonavir (Norvir).

Oral medicine for ED also might not be safe or work well if you have:

• Very low blood pressure, called hypotension, or uncontrolled high blood pressure, called hypertension.
• Severe liver disease.
• Kidney disease that needs to be treated with dialysis.
• Recent heart or blood vessel problems. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months.
• An inherited eye disease that affects the retina.
• Kidney disease that needs to be treated with dialysis.

Side effects of oral ED medicines are often mild. Common ones include:

• Flushing of the skin.
• Headache.
• Upset stomach.
• Visual changes, such as blue tinge to vision, sensitivity to light or blurred sight.
• Stuffy or runny nose.
• Back pain.
• Dizzy feeling.

Rarely, more-serious side effects can happen, including:

• Hearing loss or vision loss. Some people have suddenly lost some of their hearing or vision after taking one of these medicines. But it isn't clear whether these side effects were caused by the medicine or by health conditions that the people already had. Get medical care right away if you take oral ED medicine and have sudden loss of hearing or vision.
• An erection that doesn't go away on its own. Called priapism, this rare condition can be painful. You need to get treatment for it as soon as possible to help prevent damage to your penis. If you have an erection that lasts more than four hours, call your care team right away.

Treatments for erectile dysfunction are big business, and online scams abound. If you do buy medicines over the internet:

• Check to see if an online pharmacy is legitimate. Never order medicines from an online pharmacy that doesn't list its phone number and physical address, has prices that seem too good to be true, or tells you that no prescription is needed. Also stay away from ones that don't have a licensed pharmacist on staff to answer your questions. Some illegal businesses sell fake versions of real medicines, which can be dangerous or might not work. In the United States, the National Association of Boards of Pharmacy can tell you whether an online pharmacy is licensed and in good standing. If you live in another country, check to see if pharmacy association or regulation agencies offer services like these.
• Make sure you get the right prescription and dose. When you order medicines and when you receive them in the mail, make sure they're the exact dose and type prescribed by your doctor or other health care professional. The dose is the amount of medicine you take.
• Don't be fooled into buying "herbal viagra." Never take any medicines that claim to be the "herbal" or nonprescription version of an oral medicine for ED. These aren't an effective option, and some contain harmful substances.