Erectile Dysfunction

The PDE5 inhibitors: Viagra, Levitra, and Cialis

Approved by the FDA in 1998, sildenafil (Viagra) revolutionized the way we think about — and treat — ED, largely because it is effective and easy to use. In 2003, the FDA gave its stamp of approval to two closely related drugs, vardenafil (Levitra) and tadalafil (Cialis). Five years later, in January 2008, the FDA took the further step of approving low-dose Cialis for daily use, an option that — theoretically, at least — makes sex a possibility without advance planning.

All three drugs work in a similar fashion, by affecting the normal physiology of the penis. In particular, they block PDE5, an enzyme that breaks down the erection-producing chemical cyclic guanosine monophosphate. This enables the penis to fill with blood and to stay erect long enough for intercourse. Of course, it’s important to realize that none of these drugs is an aphrodisiac. You must feel sexually aroused in order for them to work.

The main differences between the three drugs have to do with timing: how quickly they begin to work, and how long their effects last The biggest divide lies between Cialis for daily use, which keeps a steady supply of the drug in the bloodstream, and the as-needed dosing offered by the original form of Cialis, as well as by Levitra and Viagra. Among the nondaily pills, Levitra may start working slightly faster than Viagra (within a half-hour instead of an hour) although the FDA says that like Viagra, it should be taken about an hour before sexual activity. Some studies suggest that Levitra may help some men who don’t respond to Viagra. And while some doctors are skeptical about this claim, there’s no harm in trying Levitra or Cialis if Viagra doesn’t work for you.

Cialis stays active in the body much longer than the other drugs. Viagra and Levitra last about four to five hours (and sometimes up to 12 hours). But with Cialis, the window of opportunity ranges from 24 to 36 hours, which is why it’s sometimes called “the weekend drug.” The low-dose daily option aims to allow a man to be prepared for sex anytime. Another difference among the three drugs is that while Viagra’s action may be delayed or impaired when taken with food (especially high-fat foods), that’s not the case with Levitra or Cialis.

Success rates

Because Viagra has been available the longest, there’s more research available on this drug. Pooled data involving studies of 6,659 men have shown that Viagra helped 83% of men have intercourse at least once. On the other hand, Viagra is not a panacea. A 2001 study of its long-term effectiveness, published in The Journal of Urology, reported a somewhat lower overall success rate (69%). The success rates for Levitra and 36-hour Cialis are similar to those for Viagra (59% and 69%, respectively).

Not as much information is available on the effectiveness of the daily Cialis pill. One small study found that effectiveness varies depending on dose — 2.5 milligrams (mg) or 5 mg — and level of ED:

• severe ED: 27% (2.5 mg) to 33% (5 mg)
• moderate ED: 56% (2.5 mg) to 61% (5 mg)
• mild ED: 73% (2.5 mg) to 82% (5 mg).

It’s worth noting that effectiveness of placebo was 57% for mild ED, 27% for moderate ED, and 9% for severe ED.

Viagra has proved beneficial for many men with ED resulting from spinal cord injuries. In one study, 83% of such men who took Viagra had improved erections. But for men with diabetes or advanced heart disease, the drug helped only about half of the time. The response was even lower — about 30% — for men who’d had radical prostatectomy.