WASHINGTON (AP) -- President Donald Trump's plan to change the pricing model for some medications is already facing fierce criticism from the pharmaceutical industry before he's even signed the executive order set for Monday that, if implemented, could lower the cost of some drugs.

Trump has promised that his plan -- which is likely to tie the price of medications covered by Medicare and administered in a doctor's office to the lowest price paid by other countries -- will significantly lower drug costs.

"I will be instituting a MOST FAVORED NATION'S POLICY whereby the United States will pay the same price as the Nation that pays the lowest price anywhere in the World," the Republican president posted on social media on Sunday, pledging to sign the order on Monday morning at the White House.

But the nation's leading pharmaceutical lobby on Sunday pushed back, calling it a "bad deal" for American patients. Drugmakers have long argued that any threats to their profits could impact the research they do to develop new drugs.

"Importing foreign prices will cut billions of dollars from Medicare with no guarantee that it helps patients or improves their access to medicines," Stephen J. Ubl, the president and CEO of PhRMA, said in a statement. "It jeopardizes the hundreds of billions our member companies are planning to invest in America, making us more reliant on China for innovative medicines."

Trump's so-called "most favored nation" approach to Medicare drug pricing has been controversial since he first tried to implement it during his first term. He signed a similar executive order in the final weeks of his presidency, but a court order later blocked the rule from going into effect under President Joe Biden's administration.

The pharmaceutical industry has argued that Trump's 2020 attempt would give foreign governments the "upper hand" in deciding the value of medicines in the U.S.

It's likely that Trump's executive order Monday will only impact drugs covered by Medicare Part B, the insurance for doctor's office visits. Medicare beneficiaries are responsible for picking up some of the costs to get those medications during doctor's visits, and for traditional Medicare enrollees there is no annual out-of-pocket cap on what they pay.

A report by the first Trump administration found that the U.S. spends twice as much as some other countries in covering those drugs. Medicare Part B drug spending topped $33 billion in 2021.

Trump has played up the announcement, saying it will save taxpayers big money.

"Our Country will finally be treated fairly, and our citizens Healthcare Costs will be reduced by numbers never even thought of before," Trump added.

But many Americans won't see the savings.

Trump's proposal would likely only impact certain drugs covered by Medicare and given in an office -- think infusions that treat cancer, and other injectables. But it could potentially bring billions of dollars in savings to the government -- not necessarily the "TRILLIONS OF DOLLARS" Trump boasted about in his post.

Medicare provides health insurance for roughly 70 million older Americans. Complaints about U.S. drug prices being notoriously high, even when compared with other large and wealthy countries, have long drawn the ire of both major political parties, but a lasting fix has never cleared Congress.

More common prescription drugs filled at a pharmacy would probably not be covered by the new order.

Trump's post formally previewing the action came after he teased a "very big announcement" last week. He gave no details, except to note that it wasn't related to trade or the tariffs he has announced for much of the world.

"We're going to have a very, very big announcement to make -- like as big as it gets," Trump said last week.

He came into his first term accusing pharmaceutical companies of "getting away with murder" and complaining that other countries whose governments set drug prices were taking advantage of Americans.

On Sunday, Trump took aim at the industry again, writing that the "Pharmaceutical/Drug Companies would say, for years, that it was Research and Development Costs, and that all of these costs were, and would be, for no reason whatsoever, borne by the 'suckers' of America, ALONE."

Referring to drug companies' powerful lobbying efforts, he said that campaign contributions "can do wonders, but not with me, and not with the Republican Party."

"We are going to do the right thing," he wrote.