A federal court ruling invalidating a section of the Food and Drug Administration's process of determining whether more than 300,000 nonprescription drugs are safe and effective could lead to the eventual modification of many drugs. But the ruling probably will not have any immediate effect on consumers.

Dr. Sidney Wolfe, head of the Ralph Nader-supported Health Research Group said that "over a period of time" drug manufacturers "would be forced to reformulate" the medicines. This would rid them of ingrediernts that fall into the FDA's Category III for which there now is insufficient evidence to determine whether the ingredients are safe and effective, Wolfe explained.

Jack Walden, a spokesman for the Proprietary Association, whose members manufacture 90 percent of non-prescription drugs marketed in the United States, said that most of the 350,000 over-the-counter drugs now sold would be modified in some way as a result of the FDA's lengthy study of the safety and effectiveness of the 1,000 active ingredients that comprise various drug compounds.

In addition, he said the products would have to be relabeled as new FDA restrictions are written limiting the claims manufacturers can make about their products.

U.S. District Senior Judge John J. Sirica ruled Monday that the FDA cannot certify as suitable for marketing any drug containing an ingredient that has not been determined to be safe and effective, that is the Category III ingredients.

But the judge stopped short of ordering the FDA to pull such drugs off the market. The FDA studies are continuing and it may be months or even years before it is determined which drugs are finally placed in Category III.

The FDA said it was studying the ruling.

Panels of doctors, pharmacologists, pharmacists, consumers and industy representatives have categorized more than half the 1,000 ingredients found in non-prescription drugs. According to the FDA, about 25 to 30 percent of the ingredients have been placed in Category III, which denotes insufficient evidence to determine whether the drug is safe. The rest have either been determined safe and effective (Category I) or unsafe and ineffective (Category II).

Wolfe estimated that half the non-prescription drugs marketed in the country contain a Category III ingredient, while the trade group's Walden said "anyone who makes such an estimate is irresponsible," and that no one knows how many drugs have Category III ingredients.

Some of the nation's best known non-prescription drugs contain a Category III ingredient, including such products as Dristan, Bayer Cold Tablets, Vicks Formula 44, Nyquil, Contac, Sine-Off and Allerest, according to the Health Research Group.

The FDA recently banned the use of methapyrilene, a cancer-causing antihistamine that had been the prime ingredient in nighttime sleep aids. Such products as Sominex, Compoz, Sleep-Eze, Excedrin P.M. and Nytol are being modified, according to the FDA. However. their new key ingredient, pyrilamine, is on the Category III list