A subject men don't talk about - even to each other.

This disorder affects some 30 million American males--yet not only do most of them not talk about it, but they are either embarrassed or afraid to seek medical help for it. Once thought to be largely confined to older men, it now affects many men in their physical prime. While there is yet no cure for most cases, there is very effective treatment for the condition. The dreaded problem is impotence.

Medically, the most common and distressing form of impotence is the inability to produce an erection or to maintain one effectively. Although there are many predisposing causes of impotence--smoking, heart disease, diabetes, high blood pressure, to name a few--the problem occurs in many otherwise healthy males, especially as they grow older.

Basically, the problem involves blood flow to the penis. Stimulated by anything normally producing sexual arousal, a complex group of venous spaces, called erectile tissue, fill with arterial blood, causing the penis to maintain rigidity as the smooth muscle of the penis contracts to hold the blood in place. If the flow of arterial blood into the penis is inadequate, the erectile tissue does not fill, or fills incompletely.

Aside from correcting any known underlying medical condition, treatment of impotence may either involve the substitution of a prosthetic device (a penile implant) for the erectile tissue or introduction of a drug that directly affects blood flow to the penis. Ideally, such a drug would be taken orally, but the only oral substances currently available--yohimbine, for example--have not proved to be useful in most instances. There is, however, a drug now available for direct introduction into the penis, which produces an erection within 5 to 20 minutes.

Called alprostadil, the drug is injected directly into the side of the penis from a syringe with a very fine needle--so fine, in fact, that most men experience no pain whatsoever from the needle prick. The duration of the erection is related to the dose of the drug injected, which must be determined by one's physician. Once the correct dose is established, the erection should last about an hour. Rarely (in about half of 1 percent of patients), erection will last longer, and if it lasts more than six hours, immediate medical attention is required. Overall, the use of alprostadil by needle injection has proved highly satisfactory.

Even more encouraging, especially for men who cannot stand the thought of using a needle, is the recent availability of alprostadil in the form of a tiny "seed" one places in the urethral opening at the tip of the penis, where it is quickly absorbed. Performance comes at a price, however, A package of six disposable injector devices, each containing one alprostadil "seed," costs in the range of $120 to $130, depending upon the dosage delivered. For most men, this is probably a small price to pay for satisfying sex.

If, on the other hand, the thought of a painless needle injection doesn't faze you, a satisfactory and much cheaper alternative that your doctor may suggest is papaverine for injection--an old-time smooth-muscle relaxant that has been used successfully in the department of urology at the Indiana University Medical Center.

Impotence is not a problem to be taken lightly. To be sure, abstaining from sexual intercourse is not necessarily hazardous to one's health. Elut for the married couple whose sex life may have dwindled away to nothing over the years because of the husband's impotence, treatment with one of these drugs may rejuvenate the marriage relationship.