Johnson & Johnson said it “has decided to pause all YouTube digital advertising globally to ensure our product advertising does not appear on channels that promote offensive content.” The company added, “We take this matter very seriously and will continue to take every measure to ensure our brand advertising is consistent with our brand values.”

Google, in response to the actions, said, “We don’t comment on individual customers, but as announced, we’ve begun an extensive review of our advertising policies and have made a public commitment to put in place changes that give brands more control over where their ads appear.”

Google has become the largest seller of advertising on the internet by pairing its vast network of content — its own and other publishers’ — with businesses large and small looking to grab eyeballs moving from traditional media to the web. The company’s ad business generated $22.4 billion in the fourth quarter of 2016, about 85 percent of the total revenue in the period by Google’s parent company, Alphabet. The biggest part of that still comes from search advertising.

While the pullback from major brands is a public relations blow, it is unclear if it will have much of an effect on that vast ad business. The underlying dynamic of advertising’s shift from TV toward the internet remains unchanged, and YouTube is still the largest player in the web video game.

Still, AT&T was one of the top five advertisers in the United States last year, spending nearly $1 billion through November, according to data from Kantar Media. Mr. Wieser said its size would certainly cause other marketers, and investors, to take note.

“Eventually, they’ll respond appropriately,” Mr. Wieser, who cut his recommendation on Google shares to hold, from buy, early this week, said of the company, citing “global repercussions” from the moves in Britain. “They’re not going to just see a significant business go away.”

The issue highlights the continuing risks companies face with programmatic advertising, which sends advertisers’ money through a complex web of agencies and third-party networks that resemble a stock exchange before ads appear. As advertisers target people based on their browser history — the reason a pair of jeans in an online shopping cart may follow a person around the web for weeks — and extend their reach to all manner of websites and videos based on how many people are tuning in, they are growing more reliant on technology companies to prevent them from showing up in the wrong places.