It was administered under his tongue from a rosehip oil bottle with a tiny oil dropper, she said.
She stopped about four days before the approved Elixinol course started.
"It was about maintaining that, as best I could, in a very stressful environment.
"It was like a little homeopathic dropper. So people spinning out about me giving a random amount, it was a small amount so he was introduced to him in his system. It didn't go down his throat, it was under the tongue.
"It wasn't anything risque or dangerous, it was pretty normal actually. Just under very abnormal circumstances," she said.
She said the treatment is exactly "what Alex would have chosen".
He was the first person in New Zealand to receive medicinal cannabis in hospital, when it was approved in June.
But Ms Renton said the medicine should be a "first line" treatment - a medical choice she is now campaigning for.
"We lost Alex but we're not going to lose him for no reason. This gives New Zealanders someone significant that they can relate to, a 19-year-old with a whole life in front of him.
"It may not have worked because it was so far down the track, but why is it ok to say yes to Alex but not to other people. It's not ok, that's not fair.
"The basis of this is freedom of choice, that's what this is about for us."
An investigation into the use of cannabis for medicinal purposes was carried out by the Ministry of Health this year.
Associate Health Minister Peter Dunne said the decision to approve the one-off use of Elixinol relied on "the dire circumstances and extreme severity of Mr Renton's individual case".
He said the decision should not be construed as setting a wider precedent as each case would be viewed on its own merits.
But in a Facebook post about Mr Dunne's comments, Ms Renton said it was a precedent-setting case.
"One family can change these laws as NZ is behind us," she wrote.
Wellington Hospital told TV3's current affairs show 3D the cannabis oil had little effect on Alex.
"At no stage did we observe any meaningful improvement in Alex's clinical state after starting the CBD oil," it said.