Abortion pills are now the most common method of ending a pregnancy in the US, accounting for more than half of all abortions in the country.

They have also become the new frontier in the US battle over abortion access.

Much of the fight is focused on mifepristone, one half of the two-pill regimen that makes up the safest and most effective mode of medication abortion.

Now, a lawsuit in Texas threatens to pull the drug off shelves nationwide.

Here is a look at the mifepristone, what it does and where it is used.

What is mifepristone?

The first of the two-pill regimen recommended by the Food and Drug Administration (FDA) to end a pregnancy, mifepristone blocks a hormone called progesterone, which helps the body maintain the inside of the uterus. Progesterone is necessary for a pregnancy to continue.

Mifepristone effectively stops the pregnancy, while the second drug, misoprostol, empties the uterus.

The US first approved mifepristone for the medical termination of pregnancy through seven weeks gestation in September 2000. In 2016, its approved use was extended to ten weeks of pregnancy.

Mifepristone is also used to treat women who have suffered miscarriages and Cushing syndrome, a hormone-related condition.

Misoprostol has been available by prescription for decades as a treatment for stomach ulcers and to manage postpartum haemorrhaging. Its non-pregnancy related uses are likely why it has not attracted the same controversy as mifepristone.

Is mifepristone safe?

For over 20 years of use, the FDA, the American College of Obstetrics and Gynaecologists (ACOG) and other mainstream medical organisations have maintained that both mifepristone and misoprostol are safe for use.

US studies say the two-step medication regime is about 95% effective in ending pregnancy and requires further medical follow-up less than 1% of the time.

Increasingly, anti-abortion campaigners have said that abortion medication, which they call "chemical abortion", is risky and ineffective, but their claims of widespread harm are not supported by leading medical organisations, such as the World Health Organisation and the American Medical Association.

The FDA has reported a total of 26 deaths associated with mifepristone since it was approved - a rate of about 0.65 deaths per 100,000 medication abortions. For comparison, the death rate associated with habitual aspirin use is about 15.3 deaths per 100,000 aspirin users.

Where is it available?

Mifepristone and misoprostol are, for now, widely available in states where abortion is legal.

The drugs' availability was expanded in April 2021, when the FDA said it would lift the in-person dispensing requirement for mifepristone for the duration of the Covid-19 pandemic. In December of last year, the FDA permanently lifted that requirement, meaning the medication was allowed to be sent by mail.

The decision was applauded by pro-choice advocates. And to some, abortion pills delivered by mail were seen as a workaround for the abortion bans that swept the country after the reversal of Roe v Wade last June.

Access was further expanded this year by another FDA change, which allowed retail pharmacies to dispense out the drug. Previously, only health-care providers were permitted to give out the drug.

But anti-abortion activists and lawmakers have pushed back.

In February, at least 20 Republican attorneys general signed letters threatening several of the nation's top pharmacies with legal action if they were to dispense mifepristone in their state.

And in Texas, anti-abortion campaigners have filed a lawsuit saying the drug is unsafe. The presiding judge, Trump-appointee Matthew Kacsmaryk, may issue an injunction, ordering the FDA to withdraw its approval. But it is still unclear if this is possible - many legal experts say an individual judge does not have unilateral authority over an FDA-approved medication.

A ruling in this case is expected soon.