The massive tax bill just approved by Congress—and headed to Trump’s desk—isn’t being called a health care bill, but it’s about to shake up the system in a huge way. Experts say it could trigger the most significant shifts since Obamacare, potentially destabilizing insurance markets and leaving millions with less coverage.

Editor’s Note: Even though this was sold as a tax overhaul, it’s really a stealth health care bomb. The bill guts a key part of the Affordable Care Act, and the fallout could mean higher premiums or lost coverage for vulnerable Americans. If you care about who gets medical care—and how much it costs—this is a big deal.