The thing about vaginal yeast infections is that they itch.

And burn.

And make you so miserable you don't even want to put on your underwear.

"They're not life-threatening, but they certainly interfere with the activities of daily living," says gynecologist Gay Guzinski.

common infection in women, medically known as "monilia vaginalis," vaginal yeast has traditionally been treated with prescription medications that are inserted into the vagina.

In the past few months, two of those prescription medicines -- "Gyne-Lotrimin" and "Monistat 7" -- have become available without a prescription, selling for $15 to $19 depending on where you buy them.

Now you can diagnose and treat yourself for less than the cost of an office visit.

The question is: Should you?

"Most women who have had the infection know the signs," says Dr. Guzinski, chief of the section of benign gynecology at the University of Maryland Medical Center. "The thought behind making it an over-the-counter medication is that it doesn't have many side effects, it isn't toxic, and it's available for people who have irritating symptoms and might not reach the doctor for a couple of days."

"If a woman has had yeast in the past and knows the symptoms, and checks with the doctor first by phone, I'd say, 'Go ahead and get the stuff,' " says Dr. Theodore Baramki, head of the division of reproductive endocrinology at Greater Baltimore Medical Center.

However, he warns, a variety of other conditions can cause vaginal discharge and genital itch; a woman who's treating herself for monilia is wasting time and money if what she's got is an allergic reaction to laundry products, for instance, or an infection by a different kind of organism.

nTC Even when you've really got monilia, non-prescription medicine can be unnecessarily costly compared to buying it through your health insurance prescription plan: "There are still a couple of [anti-yeast medications] that are only released by prescription," says Dr. Allen Rubin, an obstetrician-gynecologist. "From a financial standpoint, I recommend that women call their doctor and get a phoned-in prescription. Most doctors will do it for you if the symptoms are correct."

The symptoms, as anyone who's had it can tell you, are pretty distinctive: "If the patient says, 'I'm itching, I'm going crazy, I'm climbing the walls,' that's monilia," Dr. Baramki says.

On examination, the patient will have a cottage cheese-like discharge and an inflamed vagina. The itch itself is most intense in the external genital region, which may also be inflamed and swollen. That's why Dr. Baramki recommends anti-monilia creams rather than suppositories; you can smear it around better.

That's also why women have sometimes sought relief by cutting the crotch out of their pantyhose: Sweating into nylon causes release of formaldehyde, which further irritates the already sore flesh, Dr. Guzinski explains. Besides, she adds, "Pantyhose keep a layer of water up against the skin, and these organisms like it wet."

For women in the throes of a yeast attack, tight jeans are also inadvisable, she adds: "The presence of any clothing on any part of the body that moves can break down the skin and allow secondary infection."

Awful as a yeast infection is, the organism causing it is a fairly benign fungus that usually co-exists quite peacefully with the other organisms inhabiting the body's warm, wet, dark places. Most often of the "candida albicans" type, it's also found in the gastro-intestinal tract and the mouth (where it's known as thrush.)

"When it's just there, it isn't an infection," Dr. Guzinski says. "When it's there in overwhelming numbers, to the exclusion of other things, then it's an infection."

That disproportion can occur for several reasons:

An antibiotic -- like penicillin, erythromicin, tetracycline, or any of the cephalosporins or sulfa drugs -- that might have been prescribed for an infection elsewhere in the body can also kill the bacteria that keep candida in check. "If a woman has discharge and itch after taking an antibiotic, it's almost a given that it's a yeast infection," Dr. Rubin says.

* Hormonal changes alter the vaginal environment and make it more hospitable to yeast: Women are susceptible to outbreaks during pregnancy and while taking birth control pills, the doctors say.

* Excess sugar in the body's tissues also encourages the growtof yeast. "It lives on carbohydrates," Dr. Guzinski says. "When someone has persistent or recurrent yeast infections, the question is, 'Do you have diabetes?' "

Although candida has been found in the male partners of infected women, it is not considered a sexually transmitted disease. "The organism is pretty ubiquitous," Dr. Guzinski points out. "But treating the consort [of a woman with recurrent monilia] does not decrease the rate of re-infection in the woman. This is not to say men don't have yeast; they have it, but they don't have a vagina, so they don't have that warm, dark, moist place where it can multiply."

"As a rule we don't treat partners," Dr. Baramki agrees. "But occasionally a woman will say, 'He has a rash on his penis." This is not a venereal disease, but if he has a rash or an itch, he can use the cream too."

Some women, however, seem to re-infect themselves, for reasons that are not always understood. Doctors theorize that they may be discontinuing treatment before the organism is under control, or that the fungal spores have burrowed so deeply into the vaginal tissues that the medication doesn't reach them, or that the immune system has simply lost control.

Recurrence of infections is one reason to call your doctor rather than trying to treat it, again and again, on your own. Another reason to get professional advice is if the over-the-counter treatment doesn't cure the itch.

"Most people who try [an over-the-counter cure for monilia] are going to find they can clear up their problem," says Dr. Rubin. "But I would never suggest that someone use it for more than a few days; if there's no relief, call your physician."

Other infections

With the advent of over-the-counter yeast remedies, doctors warn that they are not appropriate for every vaginal infection.

These yeast remedies are intended "just for discharge and itching," says Dr. Allen Rubin, an obstetrician-gynecologist. "Monilia is a local irritation, with burning and itching, but not severe pain. If there's severe pain or abdominal pain, it could be herpes, chlamydia, or some other serious problem with overlapping symptoms."

In that case, he says, you need professional care.

Other situations in which you need your doctor's help include:

* Bacterial infections

Trichomoniasis is one of the most common. It's marked by a greenish-yellow discharge and itching -- but generally more of the former and less of the latter than occurs with monilia, says Dr. Theodore Baramki.

* Dermatitis

Inflammation and itching can be caused by irritation or allergic reaction to soap, detergent that hasn't rinsed out of underwear, dye or perfume in toilet paper, or some ingredient in a feminine hygiene product.

* Rare kinds of yeast

Most cases of monilia are caused by candida albicans and will respond to treatment with over-the-counter products. But there are at least two other strains of candida that may be less responsive to those medications and more responsive to others that are available by prescription, according to gynecologist Gay Guzinski.