I read this book in France, around 1990s, or early 2000s maybe. It looked new so it may have been written recently. I read it in french, but I cannot tell if it was translated from a foreign language...
I was a teenager at this time but the book was quite hard to read so likely not a young adult book.
At some point in the future, they know a drug that gives the ability to time travel in their mind, or dreams. But it was risky, because if they took too much of it, they could stay there for ever and the body might stay braindead.
A scientist of the future uses some drug to time travel and he goes in the mind of a business executive of the past (maybe the 1980s) to investigate a conspiracy in a big company.
My memory is very blurred, it was book hard to read because of time loops (like in the Groundhog Day movie) he lives some situations several times and modifies some details.