Insulin, heart treatments and antibiotics have flowed freely across many borders for decades, exempt from tariffs in a bid to make medicine affordable. But that could soon change.

For months, President Trump has been promising to impose higher tariffs on pharmaceuticals as part of his plan to reorder the global trading system and bring key manufacturing industries back to the United States. This month, he said pharmaceutical tariffs could come in the “not too distant future.”

If they do, the move would have serious — and wildly uncertain — consequences for drugs made in the European Union.

Pharmaceutical products and chemicals are the bloc’s No. 1 export to America. Among them are the weight-loss blockbuster Ozempic, cancer treatments, cardiovascular drugs and flu vaccines. Most are name-brand drugs that yield a large profit in the American market, with its high prices and vast numbers of consumers.

“These are critical things that keep people alive,” said Léa Auffret, who heads international affairs for BEUC, the European Consumer Organization. “Putting them in the middle of a trade war is highly concerning.”

European companies could react to Mr. Trump’s tariffs in a range of ways. Some pharmaceutical companies trying to dodge the tariffs have already announced plans to increase production in the United States, which Mr. Trump wants. Others could decide to move production there later.