Adds background in paragraphs 2 to 4, details on the investment in paragraphs 5 to 8

July 21 (Reuters) - Biogen BIIB.O will invest $2 billion more in its existing manufacturing plants in North Carolina, the drugmaker said on Monday, as it seeks to expand in the U.S. amid President Donald Trump's tariff threats.

Biogen joins major drugmakers such as Eli  Lilly LLY.N, Roche ROG.S and Merck, who have all announced investments in the U.S. as the industry braces for potential import levies.

Drugs have so far been exempt from Trump's reciprocal tariffs, but he has often argued that separate levies are needed to bolster domestic manufacturing and avoid relying on other countries for medicine supply.

Earlier this month, Trump said the tariffs may be delayed, but could be as high as 200%.

On Monday, Biogen said it will invest in North Carolina's Research Triangle Park (RTP), the home of its largest manufacturing plants that produce key therapies for multiple sclerosis and Alzheimer's.

The investments will expand Biogen's ability to develop and produce certain gene-targeting therapies, and add fill‑finish facilities, automation and artificial intelligence, the company said.

The drugmaker has invested about $10 billion in its North Carolina manufacturing to date.

It has seven factories in the state with an eighth expected to start working in the second half of 2025.

(Reporting by Christy Santhosh in Bengaluru; Editing by Sahal Muhammed)

((Christy.Santhosh@thomsonreuters.com;))