Q. Which is better: Viagra or Cialis?

A. The Food and Drug Administration has approved four similar drugs for erectile dysfunction, sold under such names as Viagra (sildenafil), Cialis (tadalafil), Levitra (vardenafil) and Stendra (avanafil). They all work the same way, producing an erection by increasing blood flow to the penis. Which drug is best may depend on your preference for quick onset versus long duration and your individual medical needs.

The discovery of Viagra is one of the classic stories of serendipity in drug development. In the 1980s, the pharmaceutical company Pfizer began studying sildenafil, the generic name for Viagra, as a treatment for heart disease. It didn’t work, but the men in the study developed unusually strong, long-lasting erections. The rest is history.

All four of the currently approved drugs seem to be equally good at producing erections. But it is difficult to be more definitive in comparing their efficacy, because few head-to-head studies have been conducted, a common problem with many drugs.

Since all four drugs are similarly effective, the answer to which drug is best will depend on other attributes, such as side effects, drug interactions, speed of onset and duration of action.

Side effects are similar among the four drugs. Headache, flushing and upset stomach are the most common side effects. And, in rare cases, all can cause a persistent and sometimes painful erection that will not go down, a condition called priapism that can result in permanent damage if not treated promptly.

Drug interactions are largely similar, with the most important interaction being a contraindication to nitrate-containing drugs, such as nitroglycerin, commonly used to treat angina.

Time of onset is an important difference between the drugs. While all take effect within an hour, Stendra is the fastest acting, often taking effect within 15 minutes.

Duration of action is another important difference between the drugs. Cialis is the longest acting, with a duration of up to 36 hours. Cialis’s long duration means that it can be taken once a day, eliminating the need to take a pill before each sexual encounter. Such once-a-day dosing has been reported to increase sexual self-confidence and spontaneity.

Which drug is better depends on your personal situation. For speed of onset, Stendra may be best. For long duration, and perhaps for spontaneity, Cialis may be best. As always, the answer must come from your doctor, who can assess patient-specific factors and help determine which is right for you.