The Chicago-area deaths of people who unwittingly took "Extra-Strength Tylenol" capsules tainted with cyanide poison dealt the first serious setback to a product that had been one of the most spectacular success stories in the history of marketing.

Tylenol, a pain reliever little known to the general public before 1975, is now the largest-selling brand among all health and beauty products in the world, and an important money-maker for its parent company, Johnson & Johnson, the world's largest manufacturer of health-care items.

Even though it appeared that Tylenol itself is not defective and that Johnson & Johnson was in no way responsible for the fatalities this week, some drug industry analysts said consumer fear could reverse the stunning growth that catapulted Tylenol to an estimated 37 percent share of the billion-dollar-a-year pain-reliever market.

Robert Kniffin, spokesman for the Johnson & Johnson division that manufactures Tylenol, said yesterday the impact on sales and growth was "just about the last thing on our minds. Our immediate problem is notification to consumers. This will have some long-range business impact, but there's no way to speculate on how much it will be."

Federal health officials have said they believe the cyanide contamination was the result of local tampering, not defects at the factories.

Nevertheless, Dr. Mark Novitch, deputy commissioner of the Food and Drug Administration, said it would be "a prudent move" to avoid all use of the "Extra-Strength Tylenol" capsules "until the situation becomes clear."

If such warnings trigger a large-scale consumer move away from Tylenol, the reaction could trigger a shift back to aspirin and aspirin-based pain-relievers such as Anacin, which since 1975 have fought a series of legal and advertising battles to protect their share of the market against Tylenol.

Except for Bayer aspirin, which is manufactured and marketed by Sterling Products Co., most aspirin is manufactured in powder form by two giant chemical companies, Dow and Monsanto, and marketed under house labels by drugstores and supermarkets.

Tylenol is a brand name for acetaminophen, a non-aspirin pain reliever developed by McNeil Consumer Products Co., a wholly owned subsidiary of Johnson & Johnson.

Before 1975, it had limited sales because it was advertised mostly to physicians, like a prescription drug.

Then Johnson & Johnson changed tactics and began an aggressive campaign of selling Tylenol to the public by claiming it relieved pain without causing any of the side-effects sometimes attributed to aspirin.

The first advertising campaign emphasized Tylenol's mildness.

But when sales lagged, Johnson & Johnson took a new tack, increasing the pain-relieving content of each tablet to 500 milligrams against the standard aspirin's 325 milligrams and stressing the product's "extra strength." Tylenol sales took off.

Johnson & Johnson does not report precise sales data, but industry analysts estimate Tylenol's sales at more than $300 million a year.

Its share of the pain-reliever market is believed to be more than three times that of the other leading products, Bayer Aspirin, Bufferin, Anacin and Excedrin.

Consumer Reports, in its August issue, dismissed most of Tylenol's claims.

"Acetaminophen has no advantage over aspirin as a pain killer," it said.

"Milligram for milligram, the two are virtually identical in effectiveness. The same holds true for fever reduction. Both are equally effective."

But similar criticisms in the past have not derailed the Tylenol express.

For the first six months of this year, Johnson & Johnson reported worldwide sales of all its products, from Band-Aids to sophisticated medical instruments, of $2.9 billion, up 8.7 percent from the same period in 1981, and profits of $297.5 million, up 19.5 percent.

Tylenol is believed to account for about 6 percent of those profits.

In an industry where a 1-percentage-point swing in market share means about $7 million in factory sales, the question before drug-industry analysts yesterday was whether consumers would rally to Tylenol and its sales growth will resume after the cyanide episode passes or whether there will be a long-term negative effect. Opinion was divided.

"This is the country's leading pain killer, and until this thing broke the growth was likely to continue," said Harvey B. Storch, an analyst with Fahnestock & Co.

"This is not something that's a result of the production process, and if you don't have any more discoveries there will be only a short-term effect. If we've heard the worst, I don't think it will have any long-term impact."

He pointed out that Johnson & Johnson stock rallied yesterday after a 3-point drop on Thursday, when it was the most actively traded issue on the New York exchange. The closing price yesterday was $43 7/8, up 3/4.

Many analysts agreed with Storch's assessment, but there were also skeptics who said the damage to Tylenol's reputation among consumers could be permanent.

"It's just a damn shame," said Robert Benezra of Alex Brown & Co., "because these fellows have always been immaculate, they have never deviated from the highest standards. I think the image has been tarnished, rightly or wrongly. Human nature being what it is, the net effect is that the company will have lost its upward momentum."

He said the largest marketers of aspirin will probably tailor their advertising "to come up with some imaginative, good-taste way to capitalize on what has occurred here."