The U.S. Food and Drug Administration is tightening oversight of imports of GLP-1 drug ingredients, used for weight loss and diabetes, amid concerns that many of the items may be adulterated and pose a safety risk.

The move comes as the FDA cracks down on unapproved compounded drugs, which had filled the gap during shortages of patented treatments. Soaring demand for Novo Nordisk and Eli Lilly's obesity drugs has fueled a booming global market for cheaper, and sometimes even counterfeit versions.

The health regulator said it had previously identified serious concerns with compounded weight-loss drugs, including dosing errors, use of unapproved salt forms and side effects, with some requiring hospitalization.

As part of its heightened scrutiny, the FDA sent an import alert on Friday authorizing detention without physical examination (DWPE) at U.S. ports. This will allow the agency to seize shipments that appear to be non-compliant with federal requirements, without inspecting them on arrival.

Importers must now provide proof of quality and compliance before their goods are released.

The FDA evaluated 48 manufacturing sites outside the U.S. and found 21% of them as non-compliant.

The regulator will also maintain a separate "green list" — identifying firms or products that have resolved compliance concerns and are not subject to automatic detention at the border.

Under the "green list," the agency identified sources in Belgium, Canada, China and India, among others, meeting its criteria, and hence are excluded from DWPE.