For the second time in less than four years, an apparently isolated criminal act turned Tylenol into the type of household word that terrifies sellers and buyers alike.

"It's a kind of terrorism," said Richard Levy, vice president of the National Pharmaceutical Council, which represents 29 large drug companies, including Tylenol's embattled maker, Johnson & Johnson.

But consumers worried about the safety of over-the-counter capsules in the wake of the latest Tylenol crisis should keep the risk in perspective and remember that alternatives exist, he and other experts said.

"The risk of dying in a car accident with an unbuckled seat belt," said Dr. William Beaver, professor of pharmacology at Georgetown University Medical Center, "is probably about a million times the risk of dying of cyanide poisoning in a Tylenol capsule." He has a bottle of 500 Extra-Strength Tylenol capsules on the shelf at home and no intention of throwing it out.

Tablets -- or the newer "caplet" pills -- offer a safer alternative for consumers afraid to take capsules, Beaver said.

"Have you ever tried to stuff a tablet with something?" he said. "For all practical purposes, it's nearly impossible to poison a tablet."

Consumers should carefully inspect all medication packages, containers and the pills themselves, he said, and report any evidence of tampering.

"The consumer has to get into the act," agreed the pharmaceutical council's Levy. "When people buy canned goods they look at the cans to make sure they're not bent. What we may have to do is develop a similar consciousness about medications."

Every Halloween, he pointed out, parents alarmed by reports of tampering keep their children home -- or probe the treats they bring home to make sure there are, for example, no razor blades in the apples.

"The difference," Levy said, "is that Halloween is optional, but some medications are not. Tylenol may be optional , but prescription medications usually aren't."

Prescription medicines, which are stored behind the pharmacist's counter, are more secure than those sold over the counter, Levy said, "but somebody who really wants to adulterate these things could do so."

In the latest Tylenol incident, a 23-year-old New York woman died Feb. 8 after taking two Extra-Strength Tylenol capsules that had been laced with potassium cyanide. A second bottle of poisoned Tylenol capsules -- manufactured at a different plant -- turned up five days later in the same town.

Appearing on the "Donahue" television program last week, Johnson & Johnson chairman James E. Burke said a computer analysis by the company concluded that the odds against two bottles of capsules being contaminated at separate plants and then ending up in stores within two blocks of each other were more than 10 billion to 1.

"We do not have any proof it didn't happen in the plant or the warehouse," Burke said, "but all logic tells us it didn't."

"This is not contamination," said pharmacologist Beaver. "This is not some kind of a medical accident. This is premeditated murder."

Whether it's the work of a disgruntled employe, a stock market manipulator, a murderer trying to throw detectives off the scent or what Beaver calls a "mad bomber type" is anybody's guess.

Whatever the motive, the Tylenol scare is a reminder that nothing is perfectly safe -- not even a time-honored dose of two pills for a headache.

"Nothing is 100 percent tamperproof," said Burke last week after a meeting of business executives at the White House, during which President Reagan praised him for "grace under pressure."

"By any measure, nonprescription drugs are the most safely packaged of any consumer product," said the Proprietary Association, a trade group for the nonprescription drug industry, in a report last year.

The cyanide-laced Tylenol was sold in triple-sealed "tamper-resistant" packages adopted by Johnson & Johnson and the rest of the industry after the 1982 Tylenol poisonings, in which seven people in Chicago died.

"We thought we had about as good a tamper-resistant package as you could make," said James Murray, a Johnson & Johnson spokesman. Whoever poisoned the Tylenol capsules, he speculated, was "someone very unbalanced but also very clever."

Johnson & Johnson has stopped making or selling Tylenol capsules and has stepped up production of "caplets," a compromise between tablets and capsules which the company designed after the 1982 tamperings. Caplets are solid, like tablets, but they are oblong and covered with a coating to make them easier to swallow.

By the end of last year, Murray said, tablets accounted for 48 percent of Tylenol sales, capsules 30 percent and caplets 22 percent. Johnson & Johnson hopes that consumers will switch from capsules to caplets without threatening Tylenol's commanding 36 percent share of the over-the-counter painkiller market.

"We just don't want to go any longer with a product that has a shell -- that is hollow," Murray said.

Despite a widespread misconception, he said, the difference in shape and form makes no difference in the drug's "bioavailability" -- the nature and the speed of its therapeutic effect.

While some consumers find capsules easier to swallow than tablets, most of the difference between the two forms is psychological, said pharmacologist Beaver. Because capsules often are associated with prescription medicine, even over-the-counter capsules are wrongly perceived by some consumers as being more powerful than tablets.

Nor is Extra-Strength Tylenol different in formula from regular Tylenol; it just contains about half-again as much acetaminophen in each pill.

"Three regular tablets equal two Extra-Strength," Beaver said. "It's just whether you'd rather swallow two bigger things or three smaller things. It's totally irrelevant as far as therapy goes."

Other drug manufacturers have said they will not follow Johnson & Johnson's lead and stop production and sale of capsule over-the-counter drugs.

"The question," Beaver said, "is how much are you going to disrupt the whole way of doing things in this country because there's a screwball loose."

A panel of technical experts from drug companies met last week with officials from the Proprietary Association and the Food and Drug Administration to discuss ways of making capsules more tamper-resistant. The panel reaffirmed its position that the steps taken after the 1982 crisis were adequate.

"We couldn't identify any magic technology to improve on it," said John T. Walden, senior vice president of the Proprietary Association.

"The whole approach is that you don't make the package harder to open," Walden said. "You try to make the package harder to open without leaving traces that the consumer can see."