Have you wondered why sex hasn't been so terrific since your doctor prescribed antihypertensives? Or perhaps a tranquilizer, a muscle relaxant, an antidepressant, or the pill?

Did you know that some drugs can cause loss of libido, infertility, interfere with erection, ejaculation and orgasm, and trigger male breast development? Have you asked your doctor about it? Or has your doctor told you that your prescription medicine might cause problems with your sexual desire or performance?

Probably not. When it comes to drugs and sex, doctors often don't tell, and patients rarely ask.

Most people would like to know not only whether a particular drug could cause a problem with sexual functioning, but also exactly what the malfunction is, how severe it will be, how likely it is to occur, how long it will last, and if there will be permanent damage. They also want to know if there are alternative forms of therapy that might decrease the chance of adverse effects.

Some people might decide they would rather put up with the illness and its consequences, or take a reduced chance for its cure, than risk losing sexual desire or ability to perform. People should have that choice, but they rarely do.

Why don't doctors tell? For several reasons: lack of knowledge, embarrassment complexity of the sexual response (made more complicated by the addition of drugs), and the belief that if they tell their patients something might happen, it will .

Doctors know that the most powerful organ in sexual functioning is the mind; the suggestion itself may produce the problem. Said one doctor at a recent medical symposium:

"I have had men with sexual problems secondary to a drug reaction. Most commonly in my practice, any antihypertensive drug, as we all know, may be the cause of impotence in a hypertensive patient. Even though I know they are taking a drug which will cause impotence. I never forewarn them about that. I prefer to let them come to see me about it."

Until recently, human sexuality was a subject ignored in most medical schools. But even when doctors are knowledgeable about sex, few have the kind of training that allows them to feel comfortable talking to their patients about it. When doctors do mention sex, they may mystify their patients with medical jargon; patients may know only street terms they are too embarrassed to use.

Also, it's almost impossible for doctors to be fully informed about all the drugs they prescribe. Keeping up with the way drugs work, side effects, and how often and to whom those side effects occur, is a full-time job in itself. In the area of sex and medicines, it's especially complicated. There are very few drugs that produce the same effect on sexual function in every person, and specific problems usually show up in only a minority of cases.

Patients who are affected, however, may suffer in silence, believing a problem is their fault, or that it is a result of their illness, their age, or that nothing can be done anyway.

What they may not know is that if a drug-related sex problem does occur, it is rarely permanent. In most cases, sexual function can be returned to normal by a simpled adjustment of the dosage, or a switch to a different drug with the same therapeutic effect.

Discussing with their doctor their-sexual feelings and behavior may be particularly important for women. About half are considered to be depressed when they are menopausal, and many doctors prescribe antidepressant drugs.

Because there is so little in medical literature about the effect an adverse effect, or may associate a sexual change with the woman's depression -- rather than the drug. There is no reason to believe, however, that anti-depressants work any differently in women than in men, or that sexual dysfunction should occur less often in women than in men.

Some categories of prescribed drugs are more likely to cause problems with libido and sexual functioning than others, especially those for high blood pressure, mental problems, and contraception.

Here is a rundown, with brand names in upper case, generic in lower case. Remember, the problems affect a small percentage of patients, and are more likely to occur in older patients and those in multiple-drug therapy.

Antihypertensives: Probably more people experience an adverse effect on their sex lives from taking a drug to lower blood pressure than from any other kind of drug.

Unfortunately, many people stop taking their antihypertensive because they believe the treatment is worse than the disease. But if hypertension is not treated, it leads to stroke, kidney damage, heart problems and eye damage.

Fortunately, there are more than 30 different antihypertensive drugs on the market, and it is likely that if a problem occurs with one of them, it will not with another.

Psychotropics: Minor transquilizers (VALIUM, LIBRIUM, etc.) probably have no direct effect on sexual function, but sometimes may decrease libido indirectly by causing psychological depression.

Antidepressants: Commonly taken antidepressants which may decrease SEXUAL FUNCTION: AMITRIPTYLINE (ELAVIL, TRIAVIL), imipramine (TOFRANIL), and protriptyline (VIVACTIL).

Antipsychotic drugs: Of the two most commonly prescribed, thioridazine (MELLARIL) and chlorpromazine (THORAZINE), MELLARIL has much more effect on sexual function than THORAZINE MELLARIL interferes with ejaculation in men of all ages at almost any dose. Both may reduce libido and potency.

Oral contraceptives: Some women, especially those over age 25, are depressed by the pill, which then depresses their sexual desire. On the other hand, some women find that freedom from fear of pregnancy afforded by the pill increases their sex drive.

There are many kinds of oral contraceptives: Most have two hormones in different proportions, but a "mini-pill" has one hormone only. If you find you're depressed by the pill and still want to keep taking it, ask you doctor to prescribe a different one.

Many other drugs sexual desire and performance. In addition to prescription drugs, sexual function may be affected by over-the-counter drugs, alcohol, and other abusable substances such as heroin, marijuana, cocaine, LSD, and inhalants.

If you suspect a drug is causing a problem, don't restrict discussions about it to yourself, your partner, or friends. Do yourself a favor. Tell your doctor.