on new medicine

[Mnemon]

Hi.

Mum's on this new medicine. It'll make her better.

From the movie "A Monster Calls"

What does the part in bold mean to you? Do you find it idiomatic?

 

[probus]

Personally I would choose "Mum has started a new medication" but see nothing wrong with the usage in the movie. It is not an idiom because the words have their usual meanings.

 

[emsr2d2]

It's perfectly natural to me (BrE). We use "on" to refer to medication that we take on a regular basis.

He's on a new drug for his bipolar disorder.
What medication are you currently on?
They've put me on Filoxen to try to reduce the inflammation.