It soon will be the end of the line for "New" Coke, thanks to a longstanding ruling by the Federal Trade Commission.

Under a 1967 FTC advisory opinion, a new or substantially altered product can no longer be labeled "new" after it has been on the market for six months. Because the opinion is nonbinding, a company could choose to call its product "new" indefinitely. But if it is accused of false advertising, it knows the position that the FTC has taken in the past.

The rule may be informal, but advertisers take it seriously -- and the Coca-Cola Co. says it will too. By October, six months after the reformulated product was introduced, the company says it will drop the "New!" from the label of "New" Coke. The company has already begun dropping that designation in some parts of the country.

However, no such fiddling will be required for Coca-Cola Classic, the "old" flavor with the new name.

VACANCIES?. . . The makeup of the FTC is still in limbo, with two commissioners apparently on the way out, and no word from Congress or the White House on likely successors.

Chairman James C. Miller III, President Reagan's nominee to replace David A. Stockman as director of the Office of Management and Budget, had been told his confirmation hearings were tentatively set to begin Sept. 19. But an OMB spokesman said yesterday that no date has been set.

Until Miller is confirmed, it will not be clear which, if any, FTC staffers will move with him to OMB.

Commissioner George W. Douglas said months ago that he would resign this month, but the White House clerk's office said yesterday it had not received a resignation letter or a notice that the president has accepted one. Douglas could not be reached yesterday, and his office would not say when his final day would be. There has been some speculation that the president or Miller may ask Douglas to stay on, possibly as chairman, if Miller goes to OMB.

PRESCRIPTION DRUG ADVERTISING . . . Two FTC economists have argued in the New England Journal of Medicine that consumers might pay lower drug prices and receive more appropriate medicines if pharmaceutical companies could advertise prescription drugs directly to the public.

In an article in the Aug. 22 edition, FTC staffers Alison Masson and Paul H. Rubin said the potential benefits of such ads outweigh the drawbacks. While prescription drugs are advertised in medical journals and newsletters, the Food and Drug Administration traditionally has discouraged their direct advertising. In recent years, however, the agency has begun to reexamine that policy.

The FTC economists said opponents contend that drug prices would rise to cover the cost of the ads and that advertising would pressure doctors to prescribe medicines that their patients don't need. But the FTC staffers said that both concerns were unjustified and that the ads would serve to educate patients about new drugs and vaccines and their side effects.

CREDIT REGULATIONS . . . The FTC has recommended that the Federal Reserve Board approve proposed regulations designed to make it easier for creditors to comply with the Equal Credit Opportunity Act and easier for federal agencies to enforce it.

The law prohibits creditors from discriminating against credit applicants on the basis of sex, race, marital status, national origin, religion, age or because they receive public assistance. The FTC is the law's enforcer for most creditors other than banks.

The FTC supported proposed revisions that would permit creditors to consider age as a factor in connection with new, statistically based evaluation systems for predicting applicants' credit-worthiness.