The Pde5 Inhibitors Viagra Levitra And Cialis - Treating Erectile Dysfunction: Mens Sexual Health

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The PDE5 inhibitors: Viagra, Levitra, and Cialis

Approved by the FDA in 1998, sildenafil (Viagra) revolutionized the way we think about — and treat — erectile dysfunction, largely because it is so easy to use and effective. In 2003, the FDA approved two closely related drugs, vardenafil (Levitra) and tadalafil (Cialis), but Viagra remains the best-known and most frequently prescribed oral medication for erectile dysfunction.

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've got to feel sexually stimulated 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 (see Table 4). 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 4 to 5 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." Another minor difference 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, in the Journal of Urology, reported a somewhat lower overall success rate (69%). The success rates for Cialis and Levitra are similar to those for Viagra (59% and 69%, respectively).

Viagra has proved beneficial for most men with erectile dysfunction 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 helps only about half of the time. The response is even lower — about 30% — for men who've had radical prostatectomy.

Side effects

So far, the risks and side effects of these three drugs seem to be roughly the same. They all work by relaxing smooth muscle cells, which widens blood vessels — primarily in the penis, but also in other parts of the body. The most common side effect is a headache, which occurs in about 16% of users (see Figure 6). Other reactions include flushing, upset stomach, nasal congestion, and urinary tract infections. But when the drugs are used properly, these side effects are relatively mild, and most disappear after a few hours. In rare cases, some men experience temporary, mild visual problems, mainly seeing a blue tinge, but also increased sensitivity to light and blurred vision. However, men with retinitis pigmentosa, a rare eye disease, should be very cautious about using these medications.

Figure 6: Side effects of Viagra

In 2005, reports linked Viagra use to another rare eye disease, nonarteric anterior ischemic optic neuropathy (NAION), which can lead to blindness. However, fewer than 50 possible cases were reported as of 2006, compared to the millions of men who have used erectile drugs without problems. Still, these reports serve as a reminder to men over 50 to get regular eye check-ups and to alert a physician to any unusual vision problems after taking a PDE5 inhibitor drug.

Drug interactions

Within several hours of taking PDE5 inhibitor drugs, blood pressure tends to drop slightly: The top (systolic) number dips 8–10 mm Hg, while the bottom (diastolic) number goes down by 5–6 mm Hg. So it's very important to avoid taking these drugs with medications that contain nitrates, which also lower blood pressure. Interactions between these two types of drugs can cause life-threatening drops in blood pressure. (The nitrates and nitrites found in food don't pose a problem.) Men who take long-acting nitrates, including isosorbide dinitrate (Isordil, Sorbitrate, and others) and isosorbide mononitrate (Imdur, Ismo, and others), or who use nitroglycerine patches or paste should not take PDE5 inhibitor drugs. And they should not use short-acting, under-the-tongue tablets or a spray form of nitroglycerine within 24 hours of taking either Viagra or Levitra. For Cialis, the ban extends to 48 hours.

Men who take medications known as alpha blockers should be cautious about taking PDE5 inhibitors. These drugs, which include doxazosin (Cardura), terazosin (Hytrin), and tamsulosin (Flomax), are used to treat BPH and high blood pressure. If you take an alpha blocker, ask your doctor before taking any PDE5 inhibitor. For example, Viagra shouldn't be taken within four hours of taking an alpha blocker.

Yet another common medication that interacts with PDE5 inhibitors is cimetidine (Tagamet), used to treat severe heartburn and stomach ulcers. Tagamet slows the breakdown of Viagra, which can effectively double the level of Viagra in the bloodstream. Because of this, men who take Tagamet should start with a lower dose of a PDE5 inhibitor. No adverse effects from taking the drugs together have been reported, but the risk of side effects such as headache or flushing may be higher.

Other considerations

Although the PDE5 inhibitors are expensive (about $10–$14 per pill), many health insurance plans (including Medicare) cover them, although most have a limit of four pills per month. Perhaps even more important considerations are the sometimes complicated and unexpected challenges that these medications may present to your sexual relationship. Some partners who've grown comfortable in a relationship without intercourse may need time to shift gears, so to speak, if the drug is successful. Use of these drugs may raise other issues for couples. Should you tell your partner before or after you've started taking it? Will this put pressure on your partner to have sex? Should you take the medication and just hope your partner will be in the mood? The best way to address these questions is to talk about them with your partner. Particularly if you're in an established relationship, your partner should know that you have a prescription for one of the drugs, and the two of you can decide together how and when to use it.

Last updated:	September 05, 2008