The companies that make America's most popular non-prescription "diet pills" have been asked by the Food and Drug Administration to take their best-selling products off the market because they contain too much of a drug with potentially dangerous side effects.

The drug phenylpropanolamine, can cause sudden attack of very high blood pressure and is dangerous for anyone with heart disease or diabetes.

Two of the products being withdrawn, Diatec Diet Aid Drops and Dietic tablets, have been heavily promoted since their introduction last month by Smithkline Corp. as the medicine that "gives your diet a fighting chance."

The other products being withdrawn are: Ayds AM/PM, Appetite Suppressant Capsules, Control Drops, Extra Strength Appedrine tablets, Vita-Slim Capsules, Super Odrinex Tablets, Power-Slim Packets and Bio Slim T Time-Release Capsules.

Four other companies have not yet responded to the FDA request. The products involved are New Improved Super Strength HUNGREX Plus Tablets, Pro-Plan Timed Release Capsule Reducing Plan, Sargents Diet Formula Tablets and Fastamine Tablets.

Diet pills, drops, candies and other preparations have sales of between $110 million and $380 million a year, according to various industry estimates.

One of the high-dose products Vita-Slim capsules, has been on sale for more than four years, according to a spokesman for the Thompson Medical Co., whose products control 50 percent of the diet and market.

But FDA officials said other firms began producing high-dose diet pills following a conclusion by an FDA panel last year that phenylpropanolamine was "safe and effective" as an appetite suppressant.

That finding has since been sharply criticized by other drug and obesity experts. A report last August in The Medical Letter a prestigious weekly journal on new drugs, stated flatly that "there is no good evidence that phenylpropanolamine . . . or any other drug can help obese patients achieve long-term weight reduction."

FDA officials say the panel's tentative approval of the higher-dose drugs (which contain up to 50 milligrams of phenylpropanolamine per dose) was never officially adopted by the agency, and that 37 1/2 milligrams per dose -- or a total of 75 milligrams per day, either in two doses or a single time, release capsule -- is the maximum allowed.

The agency plans to review the research on phenylpropanolamine -- which is also an ingredient in cold and allergy medications -- after a recent study showed that a single dose can cause alarming rises in blood pressure even in young, healthy slender people.

In a study published last January 12 of 37 medical students given 85 milligrams of phenylpropanolamine suffered serious rises in blood pressure within three hours. Several students had headaches, chest pain, heart palpitations and dizziness, and three had to be given intravenous medicines to lower their blood pressure.

The Medical Letter reported that one patient died of an irregular heart rhythm after taking phenylpropanolamine while on a tranquilizer, Mellaril, and another patient suffered kidney failure after taking phenylpropanolamine with asprin and Tylenol.

A spokesman for the Thompson Medical Co., which manufactures three of the drugs being withdrawn -- Control Drops, Extra Strength Appedrine Tablets and Vita-Slim capsules -- said company officials had been under the impression that the higher dosage was allowed. Once the agency contacted them, the company moved quickly to withdraw their three high-dose products, he said.

A Smithkline spokesman said his company had introduced Dietac after the FDA's advisory panel tentatively approved the 50 milligram dose, and had been under the impression that the agency would agree to it. One form of Dietac, the time-release capsule, will remain on the market because its dosage is within the guidlines.

Dr. John W. Norcross, chairman of the FDA advisory panel that proclaimed phenylpropanolamine safe and effective as a diet aid, said his panel was impressed by various unpublished studies of the drug presented by Thompson Medical Co. representatives.

dr. Edward L. Steinberg, the company's vice-president for research and development, said yesterday that some studies showed dieters taking phenylpropanolamine lost an average of one pound a week, while dieters taking a placebo lost half a pound a week.

Dr. Jules Hirsch, a professor at Rockfeller University who has reviewed the research on phenylpropanolamine, said the drug's performance is "really not very impressive."

"It's a multi-million dollar business." he said. "The greatest danger is . . . it pulls the wool over people's eyes by making them think they can lose weight" simply by taking a pill.