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Penile injections have now largely been replaced by oral medications, specifically sildenafil. Nevertheless, injection and topical (skin) therapies employ various agents that have properties that help achieve erection, even in many men who do not succeed with sildenafil. The standard agents used in injections or topical administration include the following:

• Alprostadil.
• Phentolamine.
• Papaverine.

Although any or all of these agents are very effective, injections or other invasive methods of administration are awkward and uncomfortable. Topical forms of some of these agents are showing promise.

Treatments Using Alprostadil

Alprostadil is derived from a natural substance, prostaglandin E1, and acts by opening blood vessels. It is an effective treatment for some men. It can be administered in three ways:

• By injection into the erectile tissue of the penis (Caverject, Edex).
• By a device that administers the drug through the urethra (MUSE system).
• In a topical cream (Topiglan, Alprox-TD) applied directly to the penis. Studies are suggesting that this approach may prove to be effective and very acceptable. FDA approval is pending at this time.

Candidates. Regardless of how it is administered, alprostadil works in many men with a wide range of medical disorders related to erectile dysfunction, including the following:

• Diabetes.
• Prostate cancer treatments. Early use of alprostadil injections after treatment, particularly when followed by oral Viagra, may be very helpful for men being treated for prostate cancer.
• Men who are taking nitrates.
• Injury.
• Alprostadil is not an appropriate choice for the following individuals:
• Men with severe circulatory or nerve damage.
• Men with bleeding abnormalities or men who are taking medications that thin the blood, such as heparin or warfarin.
• Men with penile implants.

Side Effects of Most Alprostadil Methods . Certain side effects are common to all methods of administration, although they may differ in severity depending on how the drug is given:

• Pain and burning at the application site. In one study half of the men who injected alprostadil experienced some burning and pain at the injection site. (Such effects from the cream are mild to moderate in intensity.)
• Scarring of the penis (Peyronies disease), which is most likely to occur with injections.
• Sudden, low blood pressure. Symptoms include dizziness, lightheadedness, and fainting. If these symptoms occur, the man should lie down immediately with his legs raised.
• Priapism (prolonged erection). Possible with any method, but less chance with the MUSE system than with injections. If priapism occurs, applying ice for ten-minute periods to the inner thigh may help reduce blood flow. Erections that last four hours or longer require emergency care.
• Women partners may experience vaginal burning or itching. The drug may have toxic effects if it reaches the fetus in pregnant women, so men should not use alprostadil for intercourse with pregnant women without the use of a condom or other barrier contraceptive device.

In addition, each method has other specific side effects.

Injected Alprostadil. Injected alprostadil (Caverject, Edex) employs a very small needle that the man injects into the erectile tissue of his penis. About 80% of men describe the pain of administering the injection as being very mild. Edex is a newer and less expensive form of injected alprostadil. In one 12-month study of 894 patients, Edex injections achieved erections in 95% of attempts. There is some evidence that the agent may have long-term benefits on smooth muscles. Some men have even reported return to spontaneous erections after long-term use, although objective evidence has not confirmed these findings.

The drug should not be injected more than three times a week or more than once within a 24-hour period.

Specific reports of the severity of side effects using injections include the following:

• Pain and burning at the injection site. Half of men reported this side effect in one study. To help prevent this side effect, experts in one study recommended a lower starting dose of 2.5 micrograms with subsequent doses increasing by increments of 2.5 until an erection is achieved. In this study there were only two episodes of pain out of 138 injections. (Usually, patients start with a dose of 20 micrograms.)
• Priapism. Studies report that up to 4% of men using injection therapy experienced erections lasting more than four hours, but most cases resolve without treatment.
• Scarring (Peyronies disease). This occurs in almost 8% of men who use injection therapy for more than a year. Treatment can be resumed when the condition resolves.

In spite of its general success, self-injection therapy has a high dropout rate and is less likely to be used now that oral treatments are available. The primary reasons for dropping out are the following:

• Loss of interest in the procedure.
• Partner objection or relationship breakup.
• Cost.
• Spontaneous improvement in erections.
• Side effects (reported as being severe enough to withdraw by 10% of men in one study).
• Lack of effectiveness (14% in one study).

MUSE System. The MUSE system delivers alprostadil through the urethra. It works in the following way:

• The device is a thin plastic tube with a button at the top.
• The man inserts the tube into his urethral opening right after urination. (Urinating or urine leakage right after administration may reduce the amount of medication.)
• He presses the button, which releases a pellet containing alprostadil.
• The man rolls his penis between his hands for 10 to 30 seconds to evenly distribute the drug. To avoid discomfort, the man should keep the penis as straight as possible during administration.
• The man should be upright, either sitting, standing or walking for about 10 minutes after administration. By that time, he should have achieved an erection that lasts between 30 to 60 minutes. (If a man lies on his back too soon after administration, blood flow to the penis may decrease and the erection may be lost.)
• The erection may continue after orgasm.

Reported success rates have been around 50% but range widely. A 2001 study reported higher success rates with sildenafil (Viagra), and in another study, only 18% of men requested additional refills. Some experts believe that these less than optimal results may be due to the physicians failure to educate patients and their partners adequately about the procedure.

Specific reports of side effects using the MUSE system include the following:

• Burning in the urethra. Up to 31% of MUSE administrations result in a burning sensation in the urethra that can last five to 15 minutes. This pain is generally mild to moderate, however, and is not a primary reason for discontinuing.
• Penile pain. Some pain in the penis occurs in about a quarter to a third of cases; it is usually mild.
• Low blood pressure. About 3% of patients experience low blood pressure, which can cause dizziness or fainting.
• Drug interactions. Taking certain cold and allergy remedies may offset the effects of the MUSE-administered drug.
• Other side effects. Other side effects include minor bleeding or spotting, redness in the penis, and aching in the testicles, legs, and area around the anus.

The MUSE system should not be used more than twice a day and is not appropriate for men with abnormal penis anatomy.

Topical Cream. Alprostadil is being developed as a topical cream or gel (Topiglan, Alprox-TD). The cream is applied to the tip of the penis 15 minutes before intercourse. Studies are reporting an efficacy rate of 40% to 75% and no significant side effects, although some men report a temporary burning sensation at the application site. The consequences to the female partner are not known.

Injections Using Papaverine and Phentolamine

Until the introduction of alprostadil, the two drugs used for injection therapy had been papaverine (Pavabid, Cerespan) and phentolamine (Regitine). Adverse reactions are usually minor but include pain, ulcers, and prolonged erections (priapism), which sometimes require a needle to withdraw blood or another drug to reverse the process. In a 2000 study, a combination of these two drugs produced a much higher drop out than alprostadil alone or a triple combination of all three.