A Texas judge has revoked the longstanding approval for the widely used abortion drug mifepristone.
The pill has been authorised for over 20 years, and is used in more than half of US pregnancy terminations.
In a 67-page opinion, Judge Matthew Kacsmaryk, an appointee of former President Donald Trump, suspended his own ruling for seven days to allow the government time to appeal.
His decision could limit access to the drug for millions of women.
A lawsuit filed by an anti-abortion group in Texas, the Alliance Defending Freedom, argued that the drug’s safety was never properly studied.
In his ruling, Judge Kacsmaryk said the Food and Drug Administration (FDA) approval of the drug back in 2000 had violated federal rules that allow for accelerated approval of certain drugs.
The FDA spent four years reviewing mifepristone before it was approved in 2000.
The decision could further restrict access to abortion nationwide since the US Supreme Court removed constitutional protections for the procedure last year, triggering a wave of state-by-state bans.
Legal analysts said the ruling threatened to upend the entire foundation of America’s drug regulatory system.
Mifepristone, part of a two-drug regimen that induces abortions, effectively stops the pregnancy, while the second drug, misoprostol, empties the uterus.
It was first approved for the termination of pregnancy up until seven weeks gestation.
In 2016, its approved use was extended to 10 weeks of pregnancy.
Mifepristone is also used to treat women who have suffered miscarriages and Cushing syndrome, a hormone-related condition.
The FDA, the American College of Obstetrics and Gynaecologists (ACOG) and other mainstream medical organisations say both mifepristone and misoprostol are safe for use. (BBC)