Johnson & Johnson said it was launching a consultation with workers from its subsidiary Ethicon, which runs the site in Livingston.
In a statement, Johnson & Johnson’s medical devices division (JJMD) said: “JJMD initiated the consultation process with the regional and local works councils to propose our intent to close the Ethicon ‘Kirkton’ manufacturing site in Livingston, Scotland.
“This could potentially impact approximately 400 employees, pending the outcome of the consultation process.
“We have put forth these proposals in the interest of reducing complexity and increasing agility to better serve the needs of customers and patients in today’s evolving healthcare marketplace.”
It added: “We carefully assessed our options and recognize our proposed plans have the potential to significantly affect our employees, their families and the local community.
“We are committed to supporting them through the consultation process.”
Union leaders described the announcement as “heartbreaking for staff”.
Unite regional officer Derek Ormston said: “This is shocking news. Many of our members at Ethicon have given long years of service to the company, and to be rewarded in this way will be heart-breaking. There will also be feelings of uncertainty and anger.
“Unite will now consult with our members and work hard to defend their jobs throughout this consultation process.”
The Scottish Government said it was talking to Johnson & Johnson.
Keith Brown, cabinet secretary for economy, jobs and fair work, said: “I am aware that Johnson and Johnson has announced today that it is entering consultation with staff on the potential closure of its Ethicon facility in Livingston.
“This places around 400 jobs at risk and I know this will be an extremely anxious time for the company’s employees and their families. My thoughts are very much with those individuals at this worrying time.
“The First Minister, Business Minister Paul Wheelhouse and myself, have been engaging with Johnson and Johnson, along with our enterprise agencies, to explore every possible support for the business.
“That work has been detailed and intensive, looking at what we can do both to help address immediate business challenges, and to maximise the site’s future potential. Unfortunately despite our very best efforts, the company has decided to enter into consultation on possible site closure, which is hugely disappointing.”