The spermicidal contraceptive sponge, although selling so briskly in its 12-state test market that its availability to the rest of the country has been delayed by five months, is still the subject of some controversy centering on both safety and efficacy.

Approved by the Food and Drug Administration last spring, and on the market only since July, the sponge, marketed as "Today," already has passed its 2-millionth sale. It is about to move into 11 more states in the Southwest and Southeast--as far north as the Carolinas and Tennessee--and then into the Midwest and, by early next year, into the Northeast (the Washington area).

Although regarded by many as potentially the greatest innovation in birth control since the pill, the sponge was immediately controversial. Some critics, principally women's health groups, believe some of its components are dangerous or inadequately tested, and others question its effectiveness, which is probably somewhat less than the diaphragm's 85 percent effectiveness rate. (The pill is near 100 percent and the IUD only slightly less.)

But the success of the sponge seems likely for these reasons:

* If used exactly as directed, it is probably the most effective contraceptive ever sold without a doctor's prescription, required for diaphragm, IUD and pill.

* One size fits all.

* Relatively simple to use, it is much less messy than foams, jellies or creams, and, because it can be inserted in advance, does not stifle spontaneity.

The spermicidal sponge is the brainchild of Bruce W. Vorhauer, a biomedical engineer who formed his own company, VLI Corporation, to develop and market Today.

Basically, the new contraceptive is a soft, round, white sponge smaller and thicker than a diaphragm, with a polyester strap for ease of removal. The spermicidal-soaked, polyurethane material is activated when dampened with about two tablespoons of water. It is folded in half and inserted high in the vagina.

It becomes protective immediately, and continues to be for 24 hours, no matter how often intercourse occurs during that time. It should be left in for about six hours after intercourse.

Some women's and consumer groups have criticized the sponge, the FDA and VLI, citing inadequate tests and the alleged inclusion of chemicals suspected of causing cancer.

Moreover, they are concerned that its potential for promoting toxic shock syndrome was not properly explored.

In response to the criticism, the FDA has reaffirmed its finding that the sponge is safe and effective, based on five years of tests on more than 2,000 women in this country and abroad.

Both the FDA and VLI concede, however, that its potential relationship to toxic shock syndrome is unknown. (TSS is the relatively rare but potentially fatal illness caused by toxins from a common staphylococcus bacterium. About 90 percent of the cases occur in menstruating women and about 99 percent of those women were using tampons. There have been a few cases in which a barrier contraceptive like the diaphragm or a sea sponge used like a tampon have been a suspected link in a case of TSS.

The Centers for Disease Control use a ballpark figure for TSS incidence of "5 to 10 cases per 100,000 menstruating women per year."

The Today package insert emphasizes that the sponge should not be used during menstruation, noting specifically that it is not known if a relationship to TSS might exist. The insert also includes the major warning signals of TSS: a sudden fever of 102 degrees or more plus vomiting or diarrhea.

Nevertheless, Dawn Upchurch, interim director of the National Women's Health Network, says the warnings were not specific enough. "We think it is a really good idea that over-the-counter barrier contraceptives be available to young women, but that is also the target group for TSS."

Debra Gaynor, a publicist representing VLI, says that, in fact, the sponge's spermicide--nonoxynol-9--plus three acids in the sponge are "hostile to bacteria."

Also, preliminary results from a current study suggest that the sponge may exercise some protection against sexually transmitted ills like gonorrhea and herpes. According to the Medical Letter on Drugs and Therapeutics, nonoxynol-9 has been shown in laboratory tests (as opposed to clinical tests) to protect against "most sexually transmitted microorganisms," including T. pallidum, chlamydia, monilia, trichomonas as well as herpes and gonorrhea. Results of current clinical tests are expected in time for a forthcoming meeting of the American Public Health Association.

But nonoxynol-9 is itself the subject of some unease. Although it is the active spermicidal agent in most foams, creams and jellies, at least one study has linked it to birth defects in the offspring of women using products containing it. Subsequent studies have failed to establish this, although it still is under investigation.

"Today" uses one gram of nonoxynol-9 in each sponge, considered by critics to be overly large. Only about 125 milligrams, however, are released over a 24-hour period. Most of the spermicide is used to kill the sperm absorbed by the sponge. Any sperm not absorbed by the sponge are zapped by the smaller amounts of spermicide as it is released. The sponge also serves as a barrier to the cervix.

About 2 percent of users may be sensitive to the spermicide with a possibility of irritation, itching or rash.

On the whole, however, the independent letter concludes that "although probably less effective than a diaphragm, the over-the-counter contraceptive sponge is more convenient, is probably more acceptable to many women, and appears to be as safe as other spermicide-containing barrier contraceptives."

Critics also have expressed some concern that because the sponge is relatively expensive--$1 per sponge--some women might try to reuse it, compromising both its effectiveness and their own safety.

In an effort to gauge reactions and be available to women who might not want to go to doctors, VLI has established a toll-free hotline for Today customers staffed by trained professionals who answer questions or make referrals to physicians when indicated. From its opening in July through Sept. 9, it had logged 3,000 calls, with about 5 percent involving minor problems, the single largest one being removal. (The Medical Letter points out that if the strap is turned the wrong way, removal may be difficult.)

But 95 percent of the calls have centered on one thing: Where and when would it be available.

The toll-free VLI number: 800-223-2329.

For more information on concerns surrounding use of the sponge, write Women's National Health Network, 224 7th St. SE, Washington, D.C. 20003.