THIS IS THE age of instants: instant coffee, instant communications, instant gratification, instant specials, instant replay. So it should come as no surprise that someone is advertising and selling instant willpower. A natural, especially if you are trying to lose weight.

But does it work?

Today's instant willpower comes with a long chemical name -- phenylpropanolamine hydrochloride : PPA is the short term. It started out life as a nasal decongestant and is still widely used in common cold remedies. Now it's found in more than a dozen diet aids sold without prescription in supermarkets and drugstores under brand names such as Prolamine, Dexatrim, Appedrine, Ayds and Bio-Slim. In the past year there has been a surge in the sale of these products because of the Food and Drug Administration's proposal to ban the use of amphetamines for weight reduction. Even though amphetamines (known in street language as "speed"), which is habit forming and act as stimulants to the central nervous system, are available only by prescription, they have been subject to enormous abuse. PPA is not habit forming.

The drug apparently works by telling your appestat, which regulates your appetite, that you are not as hungry as you thought you were. It acts on the part of the brain called the hypothalamus, one of whose functions is to regulate the appestat. In addition PPA dulls your sensitivity to tast and smell. The pills are taken either one, two or three times a day, depending on their strength.

The drug is not supposed to have the same serious side effects amphetamines cause, at least not for otherwise normal, healthy people. For some people, however, PPA can be extremely dangerous. Questions about its safety and effectiveness have produced some very vocal critics.

So far the critics have been successfully drowned out by the proponents, especially one of the largest manufactureres of several brands of the pills, Thompson Medical Company. The company is reported to be the leader in the diet pill market with $40 million in annual sales. Said one of Thompson's critics, a medical expert who has already tangled with the company and doesn't want to again, "They are very aggressive."

Thompson, along with other diet pill manufacturers, presented unpublished studies on phenylpropanolamine to an FDA review panel on over-the-counter drugs. FDA is currently reviewing all OTC drugs for safety and effectiveness and the panel on Miscellaneous Internal Drugs gave PPA a green light. This is not the same as the Food and Drug Administration affirming the safety and effectiveness of a drug. FDA has yet to pass judgment on the panel's recommendations. As a matter of fact, it hasn't even published the recommendations for public comment even though they were presented to the agency more than a year ago.

Not only did the review panel say that PPAworks as an appetite suppressant, but according to Armand Wlech, FDA administrator of the panel, "at the final meeting [of the panel] they upped the dosage" they said was safe and effective.

The panel had originally set the limit at 100 milligrams of phenylpropanolamine per day and then changed it to 150 mg. Previously it had been set at 75 mg. "The panel probably made a mistake," said Welch, who has since retired from FDA. Suggesting an increase in safe dosage created "an unfortunate horsepower race," Welch said, with some companies vying to put out the highest dosage level. They did this despite the fact that it was illegal. t

According to a 1975 FDA regulation drugs on the market before the review began cannot up the dosage while a review is in process, in other words until FDA gives final approval. Welch said the companies which did so, "made a marketing error and took a risk."

In April of this year, FDA told these companies to stop producing the higher dose drugs. But some of them can still be found on the market. At the same time an Australian medical journal reported on an experiment in which a single dose of 85 mg of PPA send blood pressure up to abnormally high levels in normal healthy volunteers.

William Gilbertson, director of the agency's OTC drug evaluation, said the panel had not seen the study when it made its report. But the non-profit Medical Letter, which evaluated drugs, said the Australian study is just one of several that indicate there are definite hazards associated with PPA. A highly critical article, published in Aug., 1979 mentioned five references to serious side effects from the drug, including one fatal reaction when PPA was taken along with an anti-depressant medication.

Of the 15 products purchased by The Washington Post, only six make specific reference to drug interactions, warning users that if they are taking any prescription drugs or other products containing PPA they should not take the diet aid except under the advice and supervision of a physician.

The other packages simply warn users not to take the pills if they have high blood pressure, heart disease, diabetes or thyroid disease. Some also warn pregnant and nursing mothers not to use the drug and some warn people suffering from kidney disease. There is not much consistency.

Some packages warn against use if rapid pulse, dizziness or palpitations occur. Others say the medications should be stopped if nervousness or sleepiness occur. One package says, "Do not give this product to children under 12 years of age," and a little further down in the paragraph says "if you . . . are . . . under the age of 18 do not take this drug except under the advice of a physician or pharmacist."

Said one medical expert, "Most people have no idea whether they have heart disease or hypertension or not. The point is an OTC drug has been approved and said effective for a very common condition despite the fact there may be very large numbers of people who shouldn't take it."

The Medical Letter is even more critical of the panel's work: The studies on which they based their decision "are not described and their authors are not identified." According to Dr. Carl Schultz, chief of the gastriontestinal and contreceptive drugs monograph branch of OTC, "All of the recent studies on which the panel relied were submitted by manufacturers. They are unpublished and not available to the public."

The Letter also said that "the panel found the studies ' . . . defective in one or more important facets . . .' but concluded nevertheless that these unidentified defective studies, taken together, establish effectiveness of the drug."

In addition, the publication noted that in one of these studies the difference in weight loss between the women who took 25 mg of PPA three times a day and those who took the placebo was small. About half of the women in both groups lost less than four pounds in three weeks on a 1,200 calories a day.

Welch defends the panel's decision. "The data that came to the panel is really pretty good. There was a one or two pound weight loss per week." But Welch added, "the pitch to weight reduction -- 'lose weight fast' -- is wrong. wPhenylpropanlamine is an appetite surpresssant, an anorectic. The kind of person who has an overpowering hunger desire -- get him near food and he goes slurrrp -- it isn't going to help him. It does reduce the appetite but the person has to want to lose weight. It's still a crutch."

FDA's Gilbertson agrees. "They probably do help, but they aren't a miracle pill. The impression you get from TV is that you take these and do they do it, but you have to exercise and diet."

Obesity experts are not convinced. "The only satisfactory treatment for obesity is a lifelong change in patterns of food intake and physical activity. PPA doesn't do much, if anything. If they do it for very long and like all drugs they can cause trouble for some people."