Charlotte Caldwell made the trip to Toronto and back with 12-year-old Billy to get a six-month supply to treat up to 100 seizures a day, but said border officials seized the oil on Monday.
Ms Caldwell, from Castlederg in Co Tyrone, accused Home Office Minister Nick Hurd of having "likely signed my son's death warrant" before heading to a London meeting with him.
"It's Billy's anti-epileptic medication that Nick Hurd has taken away, it's not some sort of joint full of recreational cannabis," she told a press conference.
"I will just go back to Canada and get more and I will bring it back again because my son has a right to have his anti-epileptic medication in his country, in his own home.
"Let me tell you something now: we will not stop, we are not going to stop, we are not going to give up, we have love, hope, faith for our kids and we are going to continue."