A presidential tweet roiled the pharmaceutical industry on Monday, with shares of Pfizer (PFE) and Merck (MRK) both taking a hit after President Donald Trump renewed his criticism of drug prices.

"Pfizer & others should be ashamed that they have raised drug prices for no reason," Mr. Trump said in a tweet. "They are merely taking advantage of the poor & others unable to defend themselves, while at the same time giving bargain basement prices to other countries in Europe & elsewhere. We will respond!"

Pfizer & others should be ashamed that they have raised drug prices for no reason. They are merely taking advantage of the poor & others unable to defend themselves, while at the same time giving bargain basement prices to other countries in Europe & elsewhere. We will respond! — Donald J. Trump (@realDonaldTrump) July 9, 2018

Shares of Pfizer briefly turned negative after the tweet, but were up fractionally in late afternoon trading, while Merck was just a tick below break-even after coming off its early-day high.

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The Financial Times last week reported that Pfizer had hiked prices on about 100 drugs, its second such move of the year a month after Mr. Trump signaled drug companies would soon be slashing prices.

"The list price remains unchanged for the majority of our medicines. Our portfolio includes more than 400 medicines and vaccines; we are modifying prices for approximately 10 percent of these, including some instances where we're decreasing the price," a spokesperson for Pfizer emailed. "List prices do not reflect what most patients or insurance companies pay. In the first quarter of 2018 the net selling price increase was 0 percent due to the growing amount of rebates paid back to stakeholders in the biopharmaceutical supply chain."

The president has vowed multiple times that he would bring drug prices down, calling out pharmaceutical companies on social media and in political rallies.

Mr. Trump five weeks ago said drug manufacturers would be announcing "voluntary, massive" cuts in drug prices, but that hasn't yet materialized. Health and Human Services Secretary Alex Azar offered a list of proposals that could help reduce drug costs, although some of the suggestions would need legislation or regulations to bring to fruition.