The Food and Drug Administration has decided that the nation's best-selling prescription cold tablet, Drixoral, can be sold over the counter in what is believed to be the largest such product switch in history.

Drixoral is the antihistamine and decongestant that physicians prescribed most often last year for cold symptoms such as a runny nose or sneezing. FDA officials ruled last month that the tablets could be used safely without a doctor's supervision.

The change is expected to set off a financial boom for Drixoral's manufacturer, Schering Laboratories, which sold about $50 million of the tablets last year when they were available only by prescription.

"The consumer cold market is much bigger than the prescription market," said Michael Walsh, Schering's director of over-the-counter marketing. "We kind of figure now that the sky's the limit." Drixoral, a 12-hour timed-release tablet, will be competing with well-known cold medications such as Contac and Dristan.

Walsh said it is being promoted as the only over-the-counter cold medicine containing the antihistamine most often prescribed by doctors.

FDA officials portrayed the change in Drixoral's status as part of a decade-long trend toward self-medication for the common cold. Since 1972, many cold, cough and allergy remedies have been moved to the over-the-counter market after reviews of their ingredients by FDA panels.

This process, known as the "cough-cold review," cleared one of the ingredients in Drixoral for over-the-counter use several years ago. But the tablet's other key ingredient, dexbrompheniramine maleate, was not reviewed until Schering petitioned for consideration last March. The agency officially approved the tablet for the general consumer market in a letter to Schering dated Sept. 13, Walsh said.

Other former prescription drugs that gained over-the-counter status through the review process include Contac severe cold formula, Co-Tylenol and Chlor-Trimeton, according to an FDA spokesman.

The move to the general consumer market usually carries large financial advantages for drug companies, since over-the-counter sales are about six times as high as those for prescription medicines, according to an industry spokesman.

Afrin nasal spray, another Schering product, brings in more than $60 million a year on the over-the-counter market, compared to $14 million in 1975, before it was converted from a prescription drug, Walsh said.

Sales of Chlor-Trimeton, also a Schering cold medication, have moved from $5 million in 1975 when it was a prescription drug to $50 million, he added.

"We have a longstanding commitment to making safe and effective consumer products available without a prescription," said Steven Schneider, Schering's general manager. He added: "The more we learn about medical products . . . and their effects, the more we understand which can be administered by physicians and which can be used safely and effectively by educated consumers."

FDA's approval for over-the-counter sales means the agency considers a drug safe and effective, and simple enough for patients to use by reading instructions on a label, rather than relying on the advice of a physician.

Drixoral, like other antihistamines and decongestants, tends to cause drowsiness and other side effects, and should not be used with alcohol or other sedatives, according to medical reference texts. Doctors also recommend against using it while doing anything that requires mental alertness, such as driving a car or operating machinery.

These and other warnings are now enclosed in packages of the green tablets being sold in area drugstores.