American drug stores are handing out one-third fewer prescriptions for tranquilizers now than they were in the mid-1970s. Prescriptions for sleeping pills have fallen by half.

These are two examples of a major change in the way doctors are prescribing these and other drugs, according to studies at the National Institute of Mental Health.

The number of all kinds of prescriptions, from penicillin to heart medicine, has begun to drop for the first time, despite the increased number of drugs available on the market, said Dr. Sidney Wolfe of the Public Citizen Health Research Group. The decline has been about 7 percent, or 100 million prescriptions, over the past six years.

Dr. Louis Lasagna of the University of Rochester, an authority on drug prescribing practices, says the change can be attributed to worry over the negative effects of drugs, which have been widely publicized. "I think we have learned that environmental pollution can extend to drugs," said Dr. David Bennett of the American Medical Association's division of drugs.

The most dramatic decrease has been among the psychotherapeutic drugs, including tranquilizers, sleeping pills, sedatives and anti-psychotics.

Doctors are now dispensing one-third less of those drugs than they prescribed in the mid-1970s, according to data from studies by NIMH and the Institute for Research in Social Behavior in California.

Valium, the best-selling tranquilizer, has dropped from a peak of more than 62 million prescriptions in 1975 to about 31 million in 1981.

The percentage of the U.S. population between 18 and 74 years old that are using psychotherapeutic drugs has dropped from 18.1 to 15.9 percent, said Mitchell Balter of the NIMH.

Tranquilizer prescriptions dropped from a peak of 104.5 million in 1973 to 70.8 million last year. Sleeping pill prescriptions have dipped from more than 40 million in the mid-70s to 21.1 million in 1980.

Daytime sedatives, such as phenobarbital, declined from 21 million prescriptions annually in the early 1970s to 9.6 million in 1981.

"What is happening," said Dr. Eliot Heiman of the University of Arizona, who has studied the habits and attitudes of doctors as they prescribe some drugs, "is that there is a growing opinion that benzodiazepines [tranquilizers] are morally bad, that it is a sign of weakness to use them."

He said he found in a recent study of one clinic that a quarter of the doctors saying they will never prescribe tranquilizers. "Because of all the publicity about misuse--people use the word addiction--some doctors are becoming afraid to use it. They are afraid that if they give someone a prescription for the drug, he will use it forever."

Balter, who has conducted a number of studies of psychotherapeutic drug use in the past 15 years, said the decline in use of the drugs began in the middle 1970s when doctors had firsthand experience with the negative side effects of some of them.

Over the same period, Balter found a major change in attitudes of patients toward drug use. Until this last decade, he said, "Americans have not been used to the idea that there are risks and benefits to be weighed in drug-taking. Now people have become aware of the risk part."

Dr. Lester Grinspoon of Harvard University, author of "The Speed Culture" and other books on drug prescribing, said many drugs designed to affect the brain have shown similar histories: a period of excitement among doctors and patients over their novel effect, increasing use for a number of ailments, discovery of side effects and decline to far lower use.

Because this pattern has been repeated with psychoactive drugs several times in the past few decades, doctors are becoming more cautious, he said.

Bennett of the AMA added that drug prescribing is now taught differently and more cautiously in medical school, more attention is paid to the problems of prescribing during a doctor's residency training and courses in continuing medical education have alerted older doctors as well.

He said that AMA has also "made a strong attempt to push for nondrug therapy: hot baths, jogging. Sometimes just counseling in the office is all that's required to avoid prescribing drugs unnecessarily. There is counseling, behavior therapy and there are good alcohol treatment programs that don't require a lot of drugs."

Wolfe of the health research group said, "Overall, we are very optimistic about the change. There has been a tremendous turnaround in the last five to seven years." Besides the drugs intended to change mental states, he said there have been declines in use of many other drugs as well, including some pain pills such as Darvon and even antibiotics, which doctors fear are becoming less effective because overuse has allowed bacteria to develop defenses against the drugs.

The decline in prescriptions, Wolfe said, reflects "something that was not going on 20 years ago. Then, there was little information about drug reactions among doctors, and consumers had even less. The attitude was that prescribing drugs was simply a way of the doctor helping you."

Since that time, several categories of new drugs "were leapt on by doctors and patients without knowing how dangerous they were," he said.