Robert W. McDonald, Contriubting writer

Erectile dysfunction is the inability to achieve or to maintain an erection for a period of time sufficient to complete sexual intercourse. Although the available data regarding the number of men per year seeking treatment of ED varies depending on the data's source, ED is estimated to be present in some 5 percent of men under the age of 50 and in up to 25 percent of those over the age of 60, according to the American Journal of Science.

Several medical conditions -- including high blood pressure, diabetes and high cholesterol -- are known to be associated with ED. Most of those problems interfere with blood flow in the body's smaller arteries.

Many men who report sexual problems also smoke or use alcohol, and they are often present in those on certain high blood pressure or antidepressant medications. ED is also frequently seen following surgery on the prostate gland.

Three drugs are currently available for the treatment of ED, and all three belong to a class of drugs known as PDE-5 inhibitors. They exert their effects by causing a temporary buildup in a substance called nitric oxide which, in turn, causes the muscular walls of the small arteries in the penis to relax. That increases blood flow during sexual arousal.

Cialis, formally known as tadalafil, is by far the most popular ED medication. In 2008, it was responsible for 55 to 60 percent of the total worldwide sales of $2.1 billion for all ED drugs. Cialis' gained popularity because, unlike competitors Viagra and Levitra, it is the only ED medication that is approved for daily use, and its effects have been reported to last for as long as thirty-six hours.

Many men say that being able to take Cialis on a daily basis is the primary reason for selecting it over other medications.

Viagra, sildenafil, was the first medication proven to be effective in the treatment of ED. Viagra is usually taken one hour prior to the anticipated start of intercourse, and its effects usually last for some four to six hours. Viagra accounts for 20 to 25 percent of worldwide sales.

Levitra, vardenafil, also holds a 20 to 25 percent share of the worldwide market. Levitra is generally taken one hour ahead of time, and its duration of effect is the same as that found in Viagra. As with Viagra, Levitra is not recommended for routine daily use. At the retail level, Levitra is becoming more popular because it is generally about 10 to 15 percent cheaper than Viagra.

All three medications are known to cause some unpleasant, and even potentially dangerous, side effects. Among the most common are sudden decreases in blood pressure, visual disturbances and a ringing in the ears. More serious conditions include chest pain and an erection lasting beyond four hours. Each of these conditions must be treated on an emergency basis.

ED can be treated with several medications but, as with any other medical condition, using any ED drug should only be considered after discussing the advantages and potential disadvantages of each drug with a physician.