The maker of Anacin would be required to spend about $24 million to tell consumers that the best-selling analgesic is not a tension-reliever under an initial decision by a Federal Trade Commission administrative law judge.

The judge's decision, which is to be reviewed by the FTC, is the first from three cases brought more than five years ago by the commission in a broad attack on alleged false and deceptive advertising claims by makers of the leading nonprescription painkiller.

In the initial decision released yesterday, Judge Montgomery K. Hyun found that American Home Products Corp. had made false, misleading and unsubstantiated claims for Anacin and Arthritis Pain Formula.

While the order would prohibit the company from making certain claims for Arthritis Pain Formula, and allow others only if they are qualified by certain disclosures, Judge Hyun ordered a significant amount of "corrective advertising" to clear up lingering misimpressions left in consumers' minds as a result of past ad campaigns.

American Home would be required to include the statement, "Anacin is not a tension-reliever," in every Anacin and ad until it has spent about $24 million advertising the product. That amount is the average annual Anacin advertising budget for the five years ending in 1973, when Anacin's tension-relief ads stopped.

Because of the TV ads, Hyun found viewers came away with a belief that Anacin was not only a pain-reliever but "is also good for tension, nerves, stress, fatigue and depression, and helps to cope with the ordinary stresses of everyday life." The evidence presented during hearings showed those claims to be false, the judge ruled.

He also found false, unfair and unsubstantiated American Home's claim that Anacin was superior to other over-the-counter pain-killing drugs, Contrary to its claims, Anacin does not have more pain-dulling ingredients than any other over-the-counter internal analgesic, special or stronger than aspirin, and does not contain twice as much of its analgestic ingredient as other products, the judge found.

"There are other analgesic products on the market which contain as much or more pain-dulling ingredients than does Anacin," Hyun wrote. "Anacin's analgesic ingredients is not unusual, special or stronger than aspirin, since it is nothing other than aspiring.

"Anacin's only other ingredient, caffeine, is not an analgesic . . . Indeed, both aspirin and caffeine are commonplace substances, available in many products," the judge said.

The FTC's attack on over-the-counter pain-relievers dates back more than a decade. The three cases brought in 1972 did not challenge claims of the effectiveness of analgesics on pain-relievers, only claims that one product is superior to another.

Because consumers become convinced by ads that there are significant therapeutic differences among the products, they are often induced to pay more than they might have for an analgesic, FTC officials have suggested. There isn't any good evidence that significant therapeutic differences exist, they contend.

The other two cases pending at the commission challenge advertising claims made by Bristol-Myers Co. for Bufferin, Excedrin and Excedrin PM, and Sterling Drug Inc. for Bayer aspirin, Bayer children's aspirin, Cope, Vanquish and Midol.

When the commission reviews the decisions in all three cases, the newest member of the five-member agency will not be able to participate. Robert Pitofsky was director of the FTC's Bureau of Consumer Protection at the time the complaints were brought and signed all of them.