About 36 million recipients of Social Security benefits will get a free piece of advice from the Food and Drug Administration with their checks this month: start asking more questions about prescription drugs.

The FDA, shocked by studies finding that up to 50 percent of all prescription drugs were being taken improperly, says it also will contact 125,000 physicians across the nation, warning them that patients apparently know a lot less about their medicine than doctors think they do.

The FDA said it is starting its campaign with the Social Security mailing "because the elderly are the biggest users of prescription drugs." An added advantage is that the mailing won't cost the FDA anything, because the Social Security Administration already had planned to put an insert in the July checks to explain some changes in the Social Security system.

The FDA is working with the National Council on Patient Information and Education, a private organization that is trying to prove that voluntary efforts by drug manufacturers and health-care groups can get the word out about prescription drugs without Uncle Sam stepping in with a regulation.

The council was established after the Reagan administration killed a proposed regulation that would have required the distribution of informational material with 10 of the most commonly prescribed classes of drugs.

The FDA is taking action after analyzing the results of two nationwide surveys of patients, doctors and pharmacists. What the surveys found was a classic failure to communicate.

In the first survey, only 2 to 4 percent of patients said they ask questions about their prescriptions while in the doctor's office. In the second survey, doctors said they figured patients were getting enough information because so few questions were asked.

FDA officials said many drugs are being taken in ways that could render them ineffective or unsafe.

In some cases, patients stop taking the drugs when they start feeling better. Yet some antibiotics are effective only if the whole prescription is taken, and drugs prescribed for high blood pressure usually are intended to be taken for long periods of time.

In other cases, patients on the mend will "save" their prescription drugs to take if symptoms recur, or will give them to a friend who appears to have the same symptoms. Some take the drug in too small or too large a dose, or ignore labels that warn not to take it with certain foods, such as milk, or on an empty stomach.