Restrictions on payments to foreign partners may jeopardize studies of cancer and other conditions that would benefit Americans.

One of the studies threatened by a new administration policy regarding international research is focused on prolonging the lives of women with the most common kind of breast cancer.

Up to one in five women with estrogen-fueled breast cancer experiences a life-threatening recurrence after being in remission for 10 years or even longer. If those women could be identified in advance, doctors can treat them before the cancer comes back. The medicines already exist.

But the research project, conducted with scientists in Denmark, may well be shut down. The National Institutes of Health said earlier this month that it will stop awarding grants to scientists if the projects include making a payment, called a sub-award, to a foreign collaborator.

The policy jeopardizes thousands of active international research projects that rely on partnerships with scientists and universities in other countries.

Federal health officials said they had made the change because inconsistencies across internal systems and databases make it difficult to accurately track the sub-award payments, a problem documented in several reports by the Government Accountability Office.

“This lack of visibility is unacceptable and is exactly why radical change is needed,” an N.I.H. official said in an email, even though stricter monitoring requirements were put in place in 2023, after a critical G.A.O. report.