WASHINGTON - US President Donald Trump on Thursday sent 17 letters to pharmaceutical companies' CEOs in an attempt to pressure them to lower drug prices within 60 days, threatening to take action if they refuse to comply.

On Truth Social, Trump posted individual letters to 17 drugmakers, including Eli Lilly, Pfizer and Merck, asking the companies to take steps to cut US drug prices.

The letters were sent after Trump signed an executive order in May to reinstate a policy called the "most favored nation" plan, which seeks to reduce drug prices by linking the cost of certain medications in the United States to much lower prices in other developed countries.

"Most proposals my Administration has received to 'resolve' this critical issue promised more of the same: shifting blame and requesting policy changes that would result in billions of dollars in handouts to industry," Trump said in the letters.

"Moving forward, the only thing I will accept from drug manufacturers is a commitment that provides American families immediate relief from the vastly inflated drug prices and an end to the free ride of American innovation by European and other developed nations," he said.

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Trump also warned that if these pharmaceutical companies refuse to comply, "we will deploy every tool in our arsenal to protect American families from continued abusive drug pricing practices," though he did not specify what measures would be taken.

Right now, brand-name drugs in the United States are up to three times more expensive on average than anywhere else for the identical medicines, according to the letters.

Shares of major drugmakers fell following the announcement. Both Eli Lilly and Pfizer dropped over 2 percent Thursday, while Merck's shares fell more than 4 percent.