After a decade of warnings about possible side effects of oral contraceptives, use of birth control pills has begun to decline sharply throughout the country.

In 1975, retail pharmacists filled more than 64 million prescriptions for birth control pills. Last year, they filled only 49 million -- a drop of almost 24 percent in just four years.

The decline was registered despite what experts said was an increase during those years in the number of women of peak childbearing age.

Sales of Ortho-Novum, one of the most popular brands of the pill, dropped 43 percent between 1975 and 1978, according to a confidential report prepared for the pharmaceutical industry by International Marketing Services of Ambler, Pa.

"My impression from both patients an staff is that there is an increasing anxiety about the pill," said Dr. Mary Capon, director of the George Washington University student health service. "There is an increasing willingness to try mechanical means" of birth control, she said.

That "willingness to try mechanical means" has translated into an enormous jump in the numbers of prescriptions filled by drug stores for diaphragms, from 503,000 in 1975 to 1,205,000 in 1978 -- a 140 percent increase -- according to International Marketing Services. Comparable figures for other forms of contraceptives were not available because prescriptions are not required for them.

Six years ago, about 25 per cent of all married American women of childbearing age were taking birth control pills, and as recently as 1976, that percentage has declined only to 22.5 percent, according to the National Center for Health Statistics and the Planned Parenthood Federation.

The drop in pill use varies from pharmacy to pahrmacy, but druggists in the Washingtonn metroplitan area say they are seeing declines of at least 10 percent.

"i'd say its down 10 to 15 percent," said Bob Borgatti Jr., pharmacist at the Vienna Drug Center. "The use of diaphragms has increased dramatically. tWhereas before, we only sold a few, nowe we have to keep a whole line of them."

"It used to be, you'd fill one or two diaphragm prescriptions a month. Now we fill two or three a day," said John Yurick, a pharmacist at Spring Valley Drugs in the District. Yurick said pill sales at his store have fallen off "at least 50 percent in the last year-and-a-half to two years."

According to Myra King, director of Preterm Center or Reproductive Health in the District, 42 percent of Preterm's abortion patients in 1978 chose the pill as the birth control method they would use after their abortion. c

While no hard statistics are available yet for this year, she said, "The trend in an increase in the IUD (intrauterine device) and the diaphragm. It's up to about a 10 percent increase in non-pill methods. There seems to be a high use of (oral contraceptives) continuing, but we have private physicians reporting higher use of diaphragms, condoms and foam."

"It seems there's an increase in women who choose a different method of birth control" than the pill when they first become sexually active, said Nancy Reed, director of outreach for the Community Family Planning Service, on Pennsylvania Avenue SE.

"Before, when women started on birth control they would take the pill," said Reed, "and, to a certain extent, that's still true. But it seems there's an increase in the number choosing other methods" as their initial form of birth control.

When oral contraceptives came on the market two decades ago they had three things going for them: they were convenient, they were effective and they were beleived to be safe.

The pill is still convenient. Many women take it as part of their regular morning or evening routine. Then, when they want to have intercourse, they don't have to worry about contraception.

The pill is also effective, working between 98 and 99 percent of the time if taken as prescribed every day. But in practice the pill is only about 94 percent effective, because women occasionally forget to take it, or take it a day late.

Until recently, family planning experts told women that the diaphragm was only about 85 percent effective, said Reed, "and that includes times it's left in the drawer. But in theory, if it's used right, it's 97 to 100 percent effective." And that makes it just as effective as the pill.

One woman in her late 20's who has traded in the pill for a diaphragm, said, "As you get older, you're more willing to take responsiblity for birth control. With the pill, you just took it and then put it out of your mind. But with the diaphragm, you think about it." She said many of her friends feel the same way.

And then there's the question of safety. "If you're on the pill, you kind of block out all the bad things you hear," said the woman. "But it's kind of like smoking: once you're off it, you're glad you are."

Over the past decade, there have been reports linking the pill to blood clots, some forms of cancer, gall bladder problems, liver tumors, heart disease, stroke, anemia and any number of minor problems, such as weight gain.

There have been debates over the accuracy of some of the studies. But here is very little question today that pill users face a greater risk of developing strokes, blood clots and heart conditions than women who don't use the pill. And heavy smokers run very serious risks using the pill.

"I'm advising patients who are 35 to start thinking about starting some other form of contraception," said a local obstetrician-gynecologist who asked that she not be named. "And by 40, I will not prescribe it.

"And any patient who smokes," the docotor said, "I tell to do one thing or the other: stop smoking or stop using the pill."

However, said the physician, "It's still our best method of preventing pregnancy" when none of these conditions is present."

Statistically, there is no question that with the exception of women over 40 who smoke, it is less risky to take the pill than it is to become pregnant and carry a baby to term.

Pill takers between the agees of 20 and 24 who smoke run a 1.2-in-100,000 chance of dying of a condition related to their pill use, according to information approved by the Food and Drug Administration. But pregnant women in that age group face a 5.2-in-100,000 risk of death from conditions related to pregnancy and childbirth.

Dr. Sidney Wolfe, director of Ralph Nader's Health Research Group, argues that one should not compare the risks of death from pregnancy and pill use, because pill use carries with it far more risk of disabling disease than does pregnancy.

There is no argument, however, that smokers over 40 run far more risks taking the pill than they do carrying a pregnancy to term. Those who take the pill run a 58.9-in-100,000 risk of death, while those in that age group who become pregnant face a 21.9-in-100,000 risk.

While there is an enormous decline in pill use, there are still a number of women who keep taking it because of their fears of pregnancy and distaste for abortion.

"I think I'm taking a lot of risks," said a 21-year old nursing student who smokes and takes the pill.

"I'm not really comfortable being on it," she said. "I'm only going to stay on it a year. But right now, I've got all these things going on in my life, school and everything. And I'm terrified of getting pregnant right now. And abortion . . . I just don't want to have to make that decision."