The mother of a teenager who has been in a coma for the last 71 days is hopeful that a controversial cannabis-based treatment will prove effective.

Associate Health Minister Peter Dunne approved the use of the cannibanoid oil for 19-year-old Alex Renton after he fell gravely ill. His family believe the oil is their last chance to improve Alex's condition.

After the first dose of the oil, Alex's mother Rose Renton said her son was now able to move.

"He moves now, which he hadn't done any of that for two months, so he's close to the surface but he's certainly not conscious and awake," she said.

Renton believes the treatment works by oiling the neurons in Alex's brain to stop his seizures.

"When you're in seizures and they're locked on, the anti-epileptic drugs can't get underneath to release them, to enable them to work and stop the seizing."