(MayoClinic.com) Viagra, Levitra or Cialis is often the first oral medication tried for erectile dysfunction. For most men who have trouble keeping an erection firm enough for sex (erectile dysfunction), these medications work well and cause few side effects.

Sildenafil (Viagra), vardenafil (Levitra or Staxyn) and tadalafil (Cialis) are all medications that reverse erectile dysfunction by increasing nitric oxide, a chemical naturally produced by your body. Nitric oxide opens and relaxes blood vessels in the penis, helping you get and keep an erection. These erectile dysfunction medications don't increase your sex drive and only cause erections when you are sexually stimulated.

Although they work in similar ways, each of these medications has a slightly different chemical makeup. These minor differences affect the way each medication works, such as how quickly it takes effect and wears off, and the potential side effects. Your doctor will consider these factors when deciding if one of these medications is a good choice for you. Your doctor will also consider any health problems you have and possible interactions with other medications you take.

Vardenafil (Levitra) can also be prescribed in a tablet that dissolves on the tongue (Staxyn).

A new medication, avanafil (Stendra), was approved in April 2012 by the Food and Drug Administration for treating erectile dysfunction. Stendra works similarly to Viagra, Levitra and Cialis.

Not all men can safely take erectile dysfunction medications. They can be dangerous if you have certain health problems or you're taking particular medications. Erectile dysfunction medications may not be safe if you have:

• Heart problems, including reduced blood flow (aortic stenosis or left ventricular outflow obstruction), heart pain (angina), abnormal heart rhythms (arrhythmia) or a recent heart attack
• High or low blood pressure that isn't controlled
• A history of stroke within the last six months
• Eye problems, such as retinitis pigmentosa, or if you have a family history of certain eye problems
• Severe liver disease, including cirrhosis
• Kidney disease that requires dialysis

Never take Viagra, Levitra or Cialis if you take nitrate drugs to treat heart pain (angina). Like Viagra, Levitra and Cialis, nitrate drugs dilate blood vessels. Their combined effects can cause dangerously low blood pressure and loss of consciousness. Medications that contain nitrates include:

• Nitroglycerin (Nitro-Bid, Minitran, others)
• Isosorbide (Dilatrate-SR, Isordil, Monoket, others)
• Illegal drugs such as amyl nitrite or "poppers"

Tell your doctor about any medications you're taking. A number of other drugs can also interact with Viagra, Levitra or Cialis. They include:

• Alpha blockers
• Antibiotics
• Anti-seizure medications
• Blood thinners
• Anti-arrhythmic heart medications

Most men who take Viagra, Levitra and Cialis aren't bothered by side effects. When side effects do occur, they can include:

• Headache
• Flushing (with Viagra and Levitra)
• Indigestion
• Stuffy or runny nose
• Back pain and muscle aches (with Cialis)
• Temporary vision changes, including "blue vision" (with Viagra and Levitra)
• Dizziness or fainting (rare)

In a small number of cases, men taking Viagra, Levitra or Cialis have reported more serious side effects:

• Hearing loss or vision loss. Some men have had sudden loss of hearing or loss of vision after taking one of these medications. However, it isn't clear whether vision or hearing loss was directly caused by taking the medication or by a pre-existing condition. If you're taking one of these medications for erectile dysfunction and have sudden loss of hearing or vision, seek medical help right away.
• An erection that doesn't go away on its own. Called priapism, this rare condition can be painful and requires medical treatment to avoid damage to your penis. If you have an erection that lasts more than four hours, seek medical attention.

Taking Viagra, Levitra or Cialis without treating the original problem may improve sexual performance. But it doesn't address the root cause of this symptom. Getting proper treatment for the underlying cause of erectile dysfunction may help prevent related health problems and improve sexual function as well. Conditions that can cause or worsen erectile dysfunction include:

• Heart disease. In some cases, erectile dysfunction is the first sign of serious heart or blood vessel problems such as hardened arteries (atherosclerosis).
• Diabetes. Over time, diabetes can damage blood vessels and nerves, making it more difficult to get or keep an erection.
• Psychological issues. Depression and anxiety can cause sexual problems and often require treatment, such as counseling or medications, to get better.
• Being overweight. Being overweight or obese can cause or worsen erectile dysfunction because of hormonal changes and related cardiovascular issues. Increased physical activity and losing weight can improve your overall health and sexual function.
• Low testosterone. Men who have low testosterone levels (hypogonadism) may need testosterone replacement therapy to increase sex drive and sexual performance.
• Lifestyle choices. Unhealthy habits such as drinking too much alcohol, smoking, using illegal drugs and not getting regular exercise can all worsen erectile dysfunction.

As with other prescription drugs, erectile dysfunction medications can be purchased over the Internet. But be careful. Products for erectile dysfunction are big business, and online scams abound. If you do purchase medications over the Internet:

• Check to see if an online pharmacy is legitimate. Never order medications from an online pharmacy if there's no way to contact the pharmacy by phone, if prices seem too good to be true or if you're told no prescription is necessary. Some illegal businesses sell counterfeit versions of legitimate medications, which can be ineffective or dangerous. In the U.S., the National Association of Boards of Pharmacy can tell you whether an online pharmacy is licensed and in good standing.
• Make sure you get the right prescription and the right dose. When you order medications — and when you receive them in the mail — make sure they're the exact dose and type prescribed by your doctor.
• Don't be fooled into buying 'herbal viagra.' Never take any medications that claim to be the "herbal" or nonprescription equivalent of Viagra, Levitra or Cialis. These aren't an effective replacement for prescription medications, and some contain harmful substances.

Occasional erectile dysfunction is a common problem, particularly as men age. Medication may not make you feel like you're 20 again, but it might help you achieve a more satisfying sex life. For some men, Viagra, Levitra and Cialis aren't a safe or effective choice, but other treatment options for erectile dysfunction may work. These include medications that are injected into the penis or put into the opening of the penis before sex, penis pumps (vacuum constriction devices), and surgically placed penis implants. Work with your doctor to find which erectile dysfunction treatment might work best for you.

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