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.
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A lawsuit filed by an anti-abortion group in Texas, the Alliance Defending Freedom, argued that the drug's safety was never properly studied.