The massive use of antibiotics in animal feeds is making bacteria more resistant, with growing risk to human health, a congressional study agency has concluded.

The health risk comes when the drugs - penicillin, tetracycline and others - are used in patients and prove ineffective, the Office of Technology Asessment, an arm of Congress, said in a report released this morning.

As any antibiotic is used more frequently, scientists explain, the more chance there is that it will knock out the predominant, susceptible strains of any bacteria. But then the resistance, once uncommon strains of bacteria can become predominant, and a once effective drug no longer works.

The agency also said the drugs are important contributors to health and weight gain in cattle, pigs, chickens and other farm animals. Forty percent of all the antibotics produced in the United States are made for farm use.

Therefore, the OTA said, Congress must decide which is more important: the economic gain in increased meat production or the risk to health as antibiotics become less effective.

In its report, the OTA said:

Antibiotics in feed contribute to a "growing pool of drug-resistant bacteria," and "physicians are now reporting reduced effectiveness" of the drugs in treating diseases.

There is no way to tell how much bacterial resistance is caused by ordinary medical use - and overuse - and how much by use in feeds. But the fact that "animals and humans can and do exchange" the bacteria that become resistant helps lead to the conclusion that drug use in feeds is "a significant contributor" to hospital ward resistance.

If effects in the hospital ward grow to the point that they become more readily measurable, "it may be too late" to correct the problem.

Most of the drugs used in feed could be replaced with alternative drugs already approved by the Food and Drug Administration.

The economic consequences of removal might still be "substantial" in the short term, with meat production dropping and meat prices increasing. The long-term consequences, while less certain, would probably be "small decreases or no changes" in meat production and "small increases in consumer prices."

Dr. Joyce Lashoff, OTA's assistant director for health, told a news briefing that OTA makes no recommendations, merely reports its findings and conclusions, and sets out various options for action.

Asked for her personal feelings, she said "everyone should read the report and come to their own conclusion," but "in view of the availability of substitutes, I don't believe the economic impact of restrictions proposed by FDA "would be great enough to offset the potential risk to human health" if the use of antibiotics remains unrestricted.

The difficulty in making risk-versus-benefit calculations, OTA said, is that the numbers involved are uncertain and represent estimates, and "no common denominator is generally acceptable for comparing human illness and death with pounds of meat."

In its report, the OTA sets out some of the most persuasive argumets yet for limiting or eliminating the use of at least some drugs in livestock.

It could also encourage FDA officials, who have been trying since 1977 to curb such use. Dr. Donald Kennedy, FDA commissioner, is expected to announce this week whether or not FDA will ban use of the synthetic female sex hormone, DES (diethyl stilbestrol), to promote growth in beef cattle.

DES is not an antibotic, but is a proven cause of cancer in the daughters of some women who took it durign pregnancy, and the cause of genital abnormalities is some of their sons. OTA said the amount left in meat may not be enough to cause human cancers, and the health risks of the antibiotics are apparently of "greater concern" than the risk of cancer from DES.

Still, OTA said, there are great uncertainties in assessing all such risks. An FDA administrative law judge recommended last September that DES use in cattle be banned.

Among antibiotics, FDA proposed in 1977 to prohibit routine use of penicilin in animal feeds and to eliminate most uses of two forms of tetracycline, the most widely used feed drug. The agency also withdrew approval of nihydrazone, one of four varieties of nitrofurans - another class of antibiotics - and proposed withdrawing the others.

All the FDA proposals are pending. Many farm state congressmen have questioned the need for any FDA action, and Senate Agriculture Committee Chairman Herman E. Talmadge (D-Ga.) requested the OTA assessment.