A desperate Perth mum is being forced to fly to the US to get treatment for her sick daughter, even though medicinal cannabis is legal in Western Australia.

Twelve-year-old Ava suffers from epilepsy, has had two brain surgeries, and without medicinal marijuana used to have up to eight seizures a day.

Her angry mother is so fed up with the difficulty in getting a prescription for her daughter that she plans to take her family to the US next month.