You can get a headache trying to decide if the medicine you bought for your toothache will give you a stomach ache.

In countless ads, the search for the painless tomorrow is as relentless as the search for a whiter, brighter wash.

It's one thing to let TV pick your soap. It's quite another to let it choose your medicine.

The competitive, conflicting and sometimes downright deceptive claims made by the purveyors of the hundreds of popular brands of nonprecription painkillers - to the tune of $100 million in sales a year - can cause a lot more grief than the discomfort for which they are taken.

But if you mess up in medicating your aches and pains you may be risking your health or even your life, according to Dr. William Beaver, professor of pharmacology at Georgetown University Medical School and a specialist in pain and its relief.

Contrary to popular belief, just because you can buy a medicine without a prescription - over the counter - does not mean it is safe.

So, what do you take? How much? How often?

The "in" controversy on the pain front this decade has been aspirin (the pain-relieving ingredient in Anacin, Empirin and Bufferin for example) versus ancetaminophen (as in Tylenol or Datril and most preparations that they are "aspirin-free").

And then there are those products which finesse the controversy by using both aspirin and acetaminophen (Excedrin or Vanquish, for example).

Both pain-relievers have been around since the late 19th century, but aspirin's supreme popularity has been challenged seiously only in the past few years.

Poor old aspirin began to get a bad name for itself as modern research techniques and sophisticated pharmaceutical equipment began to turn up some previously unknown aspirin effects. These include the discovery that the drug affected blood clotting and caused occasionally serious gastro-in-testinal bleeding.

It was also found to affect substances in the body called prostaglandins which control, among other things, the contractions of women in labor.

Some of these effects are now being tested as treatments for various medical disorders and some may turn out to be net assets as, for example, the use of aspirin as a mild anticoagulant. But the discoveries, along with new knowledge of a rare but dangerous actual allergy to aspirin, did tend to identify a body of consumers who should avoid aspirin or products containing aspirin unless under a physician's care.

People should NOT use aspirin with our first discussing it with a physician if they get asthma or have nasal polyps or if they get stuffy noses, wheezy breathing or red swelling around the face when they take aspirin. Others include people with bleeding problems, people with gastric ulcers and those taking certain drugs that interact badly with aspirin such as some gout remedies, oral antidiabetic agents or anticoagulants. Pregnant women approaching term should also clear its use with a physician.

Enter acetaminophen (Tylenol, etc., remember?) which seemed to do almost anything aspirin could do: relieve pain, reduce fever (but not , as aspirin does lessen inflammation.) And, of course, enter advertising campaigns promoting the acetaminophen products as "safer" for both adults and children and quickly matching and sometimes surpassing aspirin-product sales.

The major problem with acetaminophen, however, is that overdoses can cause irreversible, sometimes lethal damage to the liver and by the time symptoms show up it is too late for an antidote to work. Too much aspirin causes immediate symptoms - from ringing in the ears to hyperventilation and vomitting - and though it, too, is dangerous, the aspirin overdose is regarded as the lesser evil.

Beaver finds simple aspirin is still the best for almost everybody.

But how much to take? Again the advertisements are often misleading. With the proliferation of competing over-the-counter pain relievers, the confusion has been compounded by manufacturers playing games with the amounts of medicine in each tablet.

"The manufacturers' only limitation seems to be they can convince the consumer to swallow," laments Beaver.

That legendary first doctor who said "take two aspirins and call me in the morning" wasn't talking about extra-strength anything. He meant a tablet that had 5 grains (325 milligrams) of aspirin in it. "Take two" meant "10 grains," or 650 mg. But these pain relievers come now in many shapes and sizes and you run the risk of not taking enough or too much.

Some labels list "grains", some grains and milligrams and some milligrams alone.

When you get down to cases, "extra strength" on a label just means more aspirin or acetaminophen.

Then there is Anacin which claims in its ads to have "more of the ingredient doctors recommend most." This one drives Beaver right up thewall - he bounces in his chair and wavers his arm in frustration. You wonder why, because you'd used Anacin for years and found it worked better.

Sure, it did. It has 400 mg. of aspirin in each tablet whereas, remember, the usual tablet has 325. So technically the ad is correct but still, the most "deceptive," according to Beaver.

A spokesman for the company that manufactures Anacin said that current FDA standards are followed and that the label suggests one tablet every three hours as the recommended dose. He also said that a new maximum-strength Anacin will be on the market soon at 500 mg. a tablet.

The Food and Drug Administration is considering regulations to bring some order out of the current painkiller chaos. But as the proposed regulations wend their bureaucratic way through the laborious promulgation process, one does well to read the label and by no means exceed 10 or 15 grains in a four-hour period or 60 grains in any 24 hours, say the doctors.

Meanwhile, take all the ads with a grain of salt.

As Dr. Beaver says:

"I've spent my entire professional life trying to measure pain and analygesic and compare drugs. And even with large numbers of patients and having a full-time nurse-observer who gives the drugs and records the patients' responses, using all sorts of control procedures and sophisticated statistics, you often feel yourself lucky just to be able to prove that two aspirin tablets are better than a placebo."

And of course remember that these medicines are for simple aches and pains. Severe or persistent pain or fever always should go to the doctor.